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                                                                      EXHIBIT 99
 
             IMPORTANT FACTORS REGARDING FORWARD LOOKING STATEMENTS
 
     The following important factors, among others, could cause actual results
to differ materially from those indicated by forward-looking statements made in
the Company's Quarterly Report on Form 10-Q for the three months ended September
30, 1997 and presented elsewhere by management from time to time.
 
PATENT LITIGATION
 
     The biotechnology industry has been characterized by significant litigation
and interference proceedings regarding patents, patent applications and other
intellectual property rights, and many companies in the biotechnology industry
have attempted to employ intellectual property litigation to gain or preserve a
competitive advantage. For example, there has been substantial intellectual
property litigation between suppliers of erythropoietin throughout the world.
 
     The defense and prosecution of intellectual property suits and related
legal and administrative proceedings can be both costly and time consuming.
Litigation and interference proceedings could result in substantial expense to
the Company or its corporate partner and significant diversion of effort by the
Company's technical and management personnel. An adverse determination in
litigation to which the Company may become a party could subject the Company to
significant liabilities to third parties or require the Company to seek licenses
from third parties. Although a number of patent and intellectual property
disputes in the biotechnology area have been settled through licensing or
similar arrangements, costs associated with any such arrangement may be
substantial and could include ongoing royalties. Furthermore, there can be no
assurance that necessary licenses would be available to the Company or its
corporate partner or would be available on acceptable terms. Adverse
determinations in a judicial or administrative proceeding or failure to obtain
necessary licenses could prevent the Company or its corporate partner from
manufacturing and selling some or all of its products, which would have a
material adverse effect on the Company's business, financial condition and
results of operations.
 
     In April 1997, Amgen Inc. filed a civil action in the U.S. District Court
in Massachusetts against the Company and Hoechst Marion Roussel, Inc. ("HMRI"),
its collaborative partner. The complaint in the action alleges that GA-EPO and
processes for producing GA-EPO infringe on Amgen's U.S. Patent Numbers
5,547,933, 5,618,698, and 5,621,080 and requests that TKT and HMRI be enjoined
from making, using, or selling GA-EPO and that the court award Amgen monetary
damages. In May 1997, TKT and HMRI filed a "Motion To Dismiss, Or, In The
Alternative, For Summary Judgment" stating that under 35 U.S.C. sec. 271(e)(1),
TKT's and HMRI's activities do not constitute patent infringement and further
stating that "(n)either the product created by TKT nor the process used by TKT
and HMR infringes any valid claim of any of the Amgen patents in suit." In July
1997, the court denied TKT's and HMRI's motion and stated that the motion
instead should be refiled as a Motion for Summary Judgment. The Company can
provide no assurance as to the outcome of this litigation. A decision by the
court in Amgen's favor, including the issuance of an injunction against the
making, use or sale of GA-EPO by the Company and HMRI in the United States, or
any other conclusion of the litigation in a manner adverse to the Company and
HMRI, would have a material adverse effect on the Company's business, financial
condition, and results of operations. There can be no assurance that the Company
will not in the future become subject, in the United States or any other
country, to additional patent infringement claims, interferences and other
litigation involving patents, or any patents that may issue on any pending
patent applications, including Amgen patent applications.
 
     With respect to gene therapy technology, the Company requested, and the
U.S. Patent and Trademark Office (the "PTO") declared in January 1996, an
interference regarding an issued patent with broad claims to ex vivo gene
therapy. The participants in the interference are TKT, Genetic Therapy, Inc. (a
wholly-owned subsidiary of Novartis AG) and Somatix Therapy Corporation
("Somatix"). Somatix subsequently merged into Cell Genesys, Inc. With the
possible exception of the patents involved in the interference, the Company
believes its Transkaryotic Therapy technology does not infringe on patents
issued to date. The PTO proceeding will determine the patentability of the
subject matter of the interference and which of the parties first
 
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developed this subject matter. The process to resolve the interference can take
many years. The outcome of interferences can be quite variable: for example,
none of the three parties may receive the desired claims, one party may prevail,
or a settlement involving two or more of the parties may be reached. There can
be no assurance that TKT will prevail in this interference or that, even if it
does prevail, that the Company can meaningfully protect its proprietary
position. In the event TKT does not prevail in the interference, a January 1997
Federal Trade Commission ("FTC") decision would then be relevant. The FTC
accepted a consent order to resolve anticompetitive concerns raised by the
merger of Ciba-Geigy Limited and Sandoz Limited into the newly formed Novartis
AG. As part of the consent order, the constituent entities of Novartis will be
required to provide all gene therapy researchers and developers with
non-exclusive licenses to the patent upon which Novartis is involved in the
interference. The Company has entered into an agreement with Somatix under which
the Company's ability to market its non-viral gene therapy products will not be
affected should Somatix win the interference.
 
     Should any of its competitors have filed additional patent applications in
the U.S. that claim technology also invented by the Company, the Company may
have to participate in additional interference proceedings declared by the PTO,
all of which could result in substantial cost to the Company to determine its
rights or potential loss of rights.
 
PATENTS AND PROPRIETARY RIGHTS
 
     The Company's success may depend in large part on its ability to obtain
patent protection for its Gene Activation and gene therapy processes and
potential products in the U.S. and other countries and to obtain the right to
use in its potential products genes or other technology that have been or may be
patented by others. Currently, the Company has two issued patents and 21 pending
patent applications in the U.S. to protect its proprietary methods and
processes; it has also filed foreign patent applications corresponding to
certain of these U.S. patent applications. In addition, the Company has entered
into several agreements to license proprietary rights from other parties.
However, the patent situation in the field of biotechnology generally is highly
uncertain and involves complex legal, scientific and factual questions. To date
there has emerged no consistent policy regarding the breadth of claims allowed
in biotechnology patents. Accordingly, there can be no assurance that patent
applications relating to the technology used by the Company will result in
patents being issued or that, if issued, the patents will not be challenged,
invalidated or circumvented or will afford protection against competitors with
similar technology.
 
     Many biotechnology and pharmaceutical companies, universities and research
institutions, including competitors with substantial resources, have filed
patent applications and have been issued patents potentially relating to the
Company's Gene Activation and gene therapy technologies. In addition, certain
competitors have filed patent applications and have been issued patents relating
to certain methods of producing therapeutic proteins that the Company
anticipates producing using its Gene Activation technology. The Company's Gene
Activation and gene therapy technologies and potential products may be found to
conflict or be alleged to conflict with patents which have been or may be
granted to competitors, universities or others. There are a substantial number
of biotechnology patent applications under review at the PTO. Because patent
applications in the U.S. are maintained in secrecy until patents issue, the
Company cannot be certain that others have not filed or maintained patent
applications for technology used by the Company or covered by the Company's
pending patent applications or that the Company was the first to file patent
applications for such technology. Competitors may have filed applications for,
or may have received patents and may obtain additional patents and proprietary
rights relating to, compositions of matter or processes that block or compete
with those of the Company. Furthermore, as is the case with any pending patent
application, competitors may attempt to amend existing applications to claim
rights to compositions of matter or processes that may block the Company. No
assurance can be given that the Company's products or processes may not infringe
patents that may issue under pending patent applications.
 
     Although the Company has licensed proprietary rights to certain genes (for
example, for Factor VIII and Factor IX) to be used in its gene therapy products,
the Company presently has no proprietary rights to certain other genes that it
may later seek to use in its products and which may be the subject of issued
third party patents or pending patent applications. As a result, the Company may
be required to obtain licenses under
 
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third party patents in order to market certain of its products. If such licenses
are not made available to the Company on acceptable terms, the Company will not
be able to market such products. In addition, under the Company's license and
sublicense agreements, the licensors and sublicensors may terminate these
agreements upon the Company's failure to meet certain specified milestones. Any
such termination of an existing license or sublicense by any such licensor or
sublicensor, or any inability by the Company to obtain any required license,
could have a material adverse effect on the Company's business.
 
     The Company also relies upon unpatented proprietary technology, processes
and know-how, which the Company protects in part by confidentiality agreements
with its employees, consultants and certain contractors. There can be no
assurance that these agreements will not be breached, that the Company will have
adequate remedies for any breach, or that the Company's trade secrets will not
otherwise become known or be independently developed by competitors.
 
EARLY STAGE OF DEVELOPMENT; COMMERCIAL UNCERTAINTY
 
     TKT is at an early stage of development. All of the Company's potential
Gene Activation products are in research, preclinical development or early
clinical development. No revenues have been generated from product sales, and no
such revenues are expected for at least several years. The Gene Activation
products currently under development by the Company will require significant
additional development efforts, including extensive preclinical and clinical
testing and regulatory approval, prior to commercial use. There can be no
assurance that any Gene Activation products will ultimately be developed by the
Company and its corporate partners, or that, even if developed, these products
will receive regulatory approval. If approved, these products will compete with
established products of proven safety and efficacy, the manufacturers of which
can be expected to employ intellectual property challenges to commercialization
of these products. There can be no assurance that the Company's Gene Activation
products, if any, will be able to be commercialized or, if commercialized, that
they will be accepted by medical centers, hospitals, physicians or patients in
lieu of existing treatments. Accordingly, there can be no assurance that these
products can be successfully manufactured and marketed at prices that would
permit the Company and its corporate partners to operate profitably. The
Company's potential gene therapy products may be even further from commercial
introduction. Due to the early stage of development of the Company's potential
gene therapy products and the extensive research, development, preclinical and
clinical testing, and regulatory review process required before marketing
approval can be obtained, the Company cannot predict with certainty when it will
be able to commercialize any of its potential gene therapy products, if at all.
 
TECHNOLOGICAL UNCERTAINTY
 
     Gene Activation and gene therapy are new and rapidly evolving technologies.
Existing preclinical data on the safety and efficacy of proteins produced by the
Company's Gene Activation technology are limited, and the Company's Gene
Activation products have not yet been tested in humans or are in early clinical
development. The Company's potential gene therapy products are even further from
commercial introduction. While many approaches to gene therapy are being pursued
by pharmaceutical and biotechnology companies and academic institutions, there
are currently no marketed gene therapy products, and existing clinical data on
the safety and efficacy of potential gene therapy products are limited. The
potential gene therapy products currently under development by the Company will
require substantial additional development efforts, including extensive
preclinical and clinical testing and the receipt of regulatory approvals prior
to commercial introduction. For any given disease, gene therapy generally, as
well as the Company's specific approach to gene therapy, may not be efficacious
or may prove to have undesirable and unintended side effects, toxicities or
other characteristics that may prevent or limit commercial use. There can be no
assurance that the Company's products will obtain approval from the U.S. Food
and Drug Administration (the "FDA") or equivalent foreign regulatory authorities
for any indication.
 
UNCERTAINTY ASSOCIATED WITH CLINICAL TRIALS
 
     Subject to compliance with FDA regulations, TKT and its corporate partners
plan to undertake extensive clinical testing in humans to evaluate the safety
and efficacy of its Gene Activation and gene therapy products
 
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in development. The Company's potential Gene Activation products have not yet
been tested in humans or are in early clinical development. The rate of
completion of clinical trials is dependent upon, among other factors, the
enrollment of patients. Patient accrual is a function of many factors, including
the size of the patient population, the proximity of patients to clinical sites,
the eligibility criteria for the study and the existence of competitive clinical
trials. Delays in planned patient enrollment in the anticipated Gene Activation
clinical trials may result in program delays, which could have a material
adverse effect on TKT. Even if clinical trials are completed, there can be no
assurance that the Company or its partners will be able to submit a Product
License Application ("PLA") to the FDA or comparable regulatory agencies in
foreign countries on the schedule anticipated or that such applications will be
reviewed and approved by such regulatory agencies in a timely manner. Of the
gene therapy products under development at the Company, only one is in Phase I
human clinical trials. The Company currently intends to seek a collaborative
partner prior to proceeding with further clinical development of this product.
There can be no assurance that the Company will be able to obtain authorization
from the FDA for additional human clinical testing of any of its gene therapy
products currently in research or preclinical development. There can be no
assurance that any authorized clinical testing will be completed successfully
within any specified time period, if at all, with respect to any potential
product. There also can be no assurance that such testing will show any
potential product to be safe or efficacious or that any such product will be
approved by the FDA for any indication. Furthermore, the Company or the FDA may
suspend clinical trials at any time if the subjects or patients participating in
such trials are being exposed to unacceptable health risks. There can be no
assurance that the Company will not encounter problems in clinical trials which
will cause the Company or the FDA to delay or suspend clinical trials.
 
UNCERTAINTY OF GOVERNMENT REGULATORY REQUIREMENTS; LENGTHY APPROVAL PROCESS
 
     The Company's research and development, preclinical testing, clinical
trials, facilities and manufacturing and marketing of its products will be
subject to extensive regulation by numerous governmental authorities in the U.S.
and other countries. The regulatory process for new therapeutic products, which
includes preclinical and clinical testing of each product to establish its
safety and efficacy, can take many years and require the expenditure of
substantial resources. Data obtained from preclinical and clinical activities
are susceptible to varying interpretations which could delay, limit or prevent
FDA regulatory approval. In addition, delays or rejections may be encountered
based upon changes in FDA policy during the period of product development and
FDA regulatory review of each submitted License Application. Similar delays may
also be encountered and substantial resources expended in foreign countries.
There can be no assurance that even after such time and expenditures, regulatory
approval will be obtained for any Gene Activation or gene therapy products
developed by the Company. Moreover, if regulatory approval of a product is
granted, such approval may entail limitations on the indicated uses for which it
may be marketed and contain requirements for post-marketing follow-up studies.
Because gene therapy is a relatively new technology and products for gene
therapy have not been extensively tested in humans, the regulatory requirements
governing gene therapy products may be subject to substantial additional review
by various regulatory authorities in the U.S. and abroad. These requirements may
result in extensive delays in initiating clinical trials of gene therapy
products and in the regulatory approval process in general.
 
     Any of the foregoing effects of government regulation, as well as of
comparable foreign regulation, could delay the marketing of the Company's Gene
Activation and gene therapy products for a considerable or indefinite period of
time, materially increase the cost involved in developing, manufacturing and
marketing the Company's products, diminish or eliminate any competitive
advantage the Company may enjoy, or otherwise adversely affect the Company's
ability to conduct its business. Compliance with applicable government
regulations governing each of the Company's potential Gene Activation and gene
therapy products will require a significant commitment of time, money and effort
by the Company and its corporate partners with no assurances that any approval
will ultimately be granted on a timely basis, if at all.
 
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HISTORY OF OPERATING LOSSES; FUTURE CAPITAL NEEDS; UNCERTAINTY OF ADDITIONAL
FUNDING
 
     The Company has experienced significant operating losses since its
inception in 1988. As of September 30, 1997, the Company had an accumulated
deficit of $44,604,000. The Company expects that it will continue to incur
substantial losses for at least several years and expects cumulative losses to
increase as the Company's research and development efforts expand. The Company
expects that such losses will fluctuate from quarter to quarter and that such
fluctuations may be substantial. There can be no assurance that the Company will
ever achieve sales or profitability.
 
     The Company will require substantial funds to conduct research and
development (including preclinical and clinical testing) of its potential
products and to manufacture and market any products that are approved for
commercial sale. Based on its current operating plan, the Company believes that
its available cash will be adequate to satisfy its capital needs through 2001.
The Company's future capital requirements will depend on many factors, including
continued progress in its research and development programs, the magnitude of
these programs, the scope and results of clinical trials, the timing and receipt
of milestone payments, the time and costs involved in obtaining regulatory
approvals, the costs involved in preparing, filing, prosecuting, maintaining and
enforcing patent claims and other patent-related costs, competing technological
and market developments, the ability of the Company to establish and maintain
collaborative arrangements, and the cost of manufacturing and commercialization
activities. To date, the Company has not received any revenues from product
sales. The Company intends to seek additional funding through collaborative
arrangements or through public or private financings. There can be no assurance
that additional financing will be available on acceptable terms, if at all.
 
COMPETITION
 
     The field of biotechnology is new and evolving, and it is expected to
continue to undergo significant and rapid technological change. Technological
developments could result in the Company's potential products becoming obsolete.
The Company's products and technologies will be subject to substantial
competition, both from other companies in the field of Gene Activation and gene
therapy and from companies which have other forms of treatment of the diseases
targeted by the Company.
 
     The Company is initially focusing its Gene Activation efforts on
established products with proven safety and efficacy. The Company anticipates
that companies selling such products will compete vigorously. There can be no
assurance that the Company's Gene Activation products will be accepted by
medical centers, hospitals, physicians or patients in lieu of existing products,
or as to the effect of such competition on the market prices of the Company's
products.
 
     Although the Company has a major corporate partner, many of the Company's
existing or potential competitors have substantially greater product development
capabilities and financial, scientific, marketing or human resources than the
Company. Similarly, other competitors of the Company may enter into
collaborative relationships with other companies having such greater resources.
In addition, certain of these competitors have significantly greater experience
than the Company in undertaking human clinical trials of new therapeutic
products. Accordingly, other companies may succeed in developing products
earlier than the Company, obtaining FDA approvals for such products more rapidly
than the Company, or developing products that are more effective or less costly
than those proposed to be developed by the Company. Furthermore, if the Company
is permitted to commence commercial sales of products, it may also be competing
with respect to commercial manufacturing and marketing capabilities, areas in
which it has no experience.
 
NO MANUFACTURING, DISTRIBUTION OR MARKETING CAPABILITY
 
     Although the Company has a pilot gene therapy manufacturing facility and
believes it will be able to manufacture its potential products on a large scale,
the feasibility of large-scale manufacturing of such products has not been
demonstrated. If the Company is unable to develop or contract for manufacturing
capabilities on acceptable terms, the Company's ability to commercialize its
potential products would be materially adversely affected. If the Company is
delayed in establishing suitable manufacturing capabilities, the Company's
ability to conduct human clinical testing may be adversely affected, resulting
in the delay of
 
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submission of potential products for regulatory approval and initiation of new
development programs, which in turn could impair materially the Company's
competitive position and the possibility of the Company achieving profitability.
In addition, although the Company believes that its potential products will be
cost-effective, there can be no assurance that the Company will be able to
manufacture and distribute such products at a reasonable cost, that the Company
will be able to price such products competitively or, if priced competitively,
that the Company will be able to achieve margins sufficient to allow it to
achieve profitability.
 
     The Company plans to provide its gene therapy products through central
manufacturing facilities. The establishment of these facilities will require
substantial additional funds and personnel and will require compliance with
extensive regulations applicable to such facilities. There can be no assurance
that such funds and personnel will be available on acceptable terms, if at all,
or that the Company will be able to comply with such regulations at acceptable
cost, if at all. In addition, in managing this expansion the Company may
encounter unforeseen regulatory, logistical or management problems or incur
unexpected operating costs. Failure or delays in establishing these facilities,
or the incurrence of unexpected operating costs, could adversely affect the
ability of the Company to manufacture and market its gene therapy products.
Furthermore, the Company has no experience in sales, marketing or distribution.
In order to market any of its gene therapy products, the Company must develop a
marketing and sales capability, either on its own or in conjunction with others.
There can be no assurance that the Company will be able to enter into any
arrangements for the marketing of its products, that such arrangements will be
successful, or that the Company will be able to obtain additional capital and
expertise to conduct such activities independently.
 
     The Company has no manufacturing, sales, marketing or distribution
capabilities for its Gene Activation products. The Company's collaborative
partner, HMRI, is responsible for the manufacture, sales, marketing and
distribution of GA-EPO and the undisclosed second protein. With respect to
future Gene Activation products, the Company may seek collaborative partners or
may manufacture and commercialize the products on its own. There can be no
assurance that the Company will be successful in establishing such future
collaborative relationships or that the Company will be able to conduct such
activities independently.
 
DEPENDENCE ON KEY PERSONNEL
 
     The Company's success is highly dependent on the retention of principal
members of its scientific and management staff. Furthermore, the Company's
future growth will require the hiring of significant numbers of qualified
scientific and management personnel. Accordingly, recruiting and retaining such
personnel in the future will be critical to the Company's success. There is
intense competition from other companies and research and academic institutions
for qualified personnel in the areas of the Company's activities, and there can
be no assurance that the Company will be able to continue to attract and retain
on acceptable terms the qualified personnel necessary for the development of its
business.
 
DEPENDENCE ON HMRI AND OTHER COLLABORATIVE PARTNERS
 
     The Company has entered into arrangements with HMRI on two of its Gene
Activation development programs and with another corporate partner on a gene
therapy development program. Each agreement with HMRI is subject to termination
without cause on short notice under certain circumstances, and there is no
assurance that in the future either partner will not exercise its termination
rights. The Company is relying on HMRI to develop, conduct clinical trials,
obtain regulatory approval for the sale of, manufacture and market GA-EPO and
the undisclosed second protein worldwide. There can be no assurance that HMRI
will devote the resources necessary to complete development of and commercialize
these two potential products. Should HMRI fail to develop and commercialize
these two potential products, the Company's business would be materially
adversely affected.
 
     The Company's strategy for the research, development and commercialization
of certain of its potential products includes the possibility that it will enter
into various additional arrangements with corporate partners, licensors,
licensees and others. There can be no assurance that any further arrangements
will be effected in the future. Although the Company believes parties to any
existing and future arrangements, if entered into, would have economic and other
motivations to perform their contractual responsibilities in full, the amount
and
 
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timing of resources which they would devote to these activities would not be
within the control of the Company. There can be no assurance that such parties
would perform their obligations as expected or that any revenue would be derived
by the Company from such arrangements.
 
PRODUCT LIABILITY AND INSURANCE
 
     The Company's business will in the future expose it to potential product
liability risks which are inherent in the testing, manufacturing and marketing
of human therapeutic products. Although the Company has clinical trial liability
insurance for trials conducted in the U.S., the Company does not currently have
any product liability insurance, and there can be no assurance that it will be
able to obtain or maintain such insurance on acceptable terms, if at all, or
that any insurance obtained will provide adequate protection against potential
liabilities. An inability to obtain insurance at acceptable cost or otherwise
protect against potential product liability claims, in addition to exposing the
Company to significant liabilities, could prevent or inhibit the
commercialization of products developed by the Company.
 
UNCERTAINTY OF PHARMACEUTICAL PRICING AND REIMBURSEMENT
 
     The business and financial condition of pharmaceutical and biotechnology
companies will continue to be affected by the efforts of government and
third-party payors to contain or reduce the cost of health care through various
means. For example, in certain foreign markets, pricing and profitability of
prescription pharmaceuticals is subject to government control. In particular,
individual pricing negotiations are often required in each country of the
European Community, even if approval to market the drug is obtained. In the U.S.
there have been, and the Company expects that there will continue to be, a
number of federal and state proposals to implement similar government control.
In addition, an increasing emphasis on managed care in the U.S. has and will
continue to increase the pressure on pharmaceutical pricing. While the Company
cannot predict whether any such legislative or regulatory proposals will be
adopted or the effect such proposals or managed care efforts may have on its
business, the announcement of such proposals or efforts could have a material
adverse effect on the Company's ability to raise capital, and the adoption of
such proposals or efforts could have a material adverse effect on the Company's
business, financial condition and results of operations. Further, to the extent
that such proposals or efforts have a material adverse effect on other
pharmaceutical companies that are prospective corporate partners for the
Company, the Company's ability to establish corporate collaborations may be
adversely affected. In addition, in both domestic and foreign markets, sales of
the Company's products, if any, will be dependent in part on the availability of
reimbursement from third party payors, such as government and private insurance
plans. Third party payors are increasingly challenging the prices charged for
medical products and services. If the Company succeeds in commercializing
products, there can be no assurance that these products will be considered cost
effective, that reimbursement will be available, or if available, that the
payor's reimbursement policies will be adequate to permit the Company to realize
a reasonable return.