Exhibit 10.6

FIFTH AMENDMENT TO NEW LEASE

      This Fifth Amendment to New Lease ("Amendment") is entered into, and dated
for reference purposes, as of July 11, 2008 (the "Execution Date") by and
between METROPOLITAN LIFE INSURANCE COMPANY, a New York corporation
("Metropolitan"), as Landlord ("Landlord"), and PHARMACYCLICS, INC., a Delaware
corporation ("Pharmacyclics"), as Tenant ("Tenant"), with reference to the
following facts ("Recitals"):

      A. Landlord and Tenant are parties to that certain written lease comprised
of the following: that certain Lease and Lease Termination Agreement dated as of
June 14, 2000 (the "New Lease"), that certain First Amendment to New Lease dated
as of April 10, 2001 (the "First Amendment"), that certain Second Amendment to
New Lease dated as of June 29, 2001 (the "Second Amendment"), that certain Third
Amendment to New Lease dated as of February 5, 2003 (the "Third Amendment") and
that certain Fourth Amendment to New Lease dated as of August 14, 2006 (the
"Fourth Amendment") for the premises more particularly described therein of
64,776

rentable square feet ("RSF") located at 995 and 999 E. Arques Avenue, Sunnyvale,
California ("Existing Premises"). The New Lease, as amended, is referred to
herein as the "Existing Lease."

      B. Landlord and Tenant desire to surrender a portion of the Existing
Premises defined in the First Amendment as "Expansion Space A," and referred to
herein as Expansion Space A, add an expansion space to the Existing Premises
previously defined in the Second Amendment as "Expansion Space B," however for
purposes of this Amendment shall hereinafter be defined as the "Expansion Space
C" and provide for extension of the Lease Term and other amendments of the
Existing Lease as more particularly set forth below.

      NOW, THEREFORE, in consideration of the foregoing, and of the mutual
covenants set forth herein and of other good and valuable consideration, the
receipt and adequacy of which are hereby acknowledged, the parties hereto agree
as follows:

      Section 1. Scope of Amendment; Defined Terms. Except as expressly provided
in this Amendment, the Existing Lease shall remain in full force and effect.
Should any inconsistency arise between this Amendment and the Existing Lease as
to the specific matters which are the subject of this Amendment, the terms and
conditions of this Amendment shall control. The term "New Lease" as used herein
and in the Existing Lease shall refer to the Existing Lease as modified by this
Amendment, except as expressly provided in this Amendment. All capitalized terms
used in this Amendment and not defined herein shall have the meanings set forth
in the Existing Lease unless the context clearly requires otherwise.

      Section 2. Retroactive Effect. Landlord and Tenant have executed this
Amendment on the respective dates set forth below their respective signatures at
the end of this Amendment, but intend and agree that it shall be effective as of
July 1, 2008 with the same force and effect as if executed on that date.
Promptly after execution of this Amendment, Landlord shall bill Tenant for the
amount of Monthly Installments of rent and all other rent due pursuant to this
Amendment in excess of amounts actually paid by Tenant for the period from and
after the Expansion Space C Commencement Date (defined below), and Tenant shall
pay the amount due within ten (10) days after receipt of such bill. In the event
that amounts actually paid by Tenant on after the Expansion Space C Commencement
Date and before execution of this Amendment exceed the amount due pursuant to
this Amendment, Landlord shall credit any such overpayment to rent next coming
due from Tenant hereunder. Within a reasonable time after such execution,
Landlord shall refund to Tenant any excess, and Tenant shall pay Landlord any
shortage, in payments received from Tenant with respect to the period from the
Expansion Space C Commencement Date through the date of such execution in
comparison to payments due pursuant to this Amendment for the same period.

      Section 3. Surrender of 999 E. Arques Avenue.

      (a) Surrender Date.  On or before 11:59 p.m. on June 30, 2008 (the
"Surrender Date"), except as set forth in Section 3(c)(i) below, Tenant shall
vacate and deliver to Landlord exclusive possession of the portion of the
Existing Premises which is located at 999 E. Arques Avenue, Sunnyvale,
California ("Expansion Space A"), in broom clean condition, except that Landlord
and Tenant acknowledge that cubicles and office furniture currently existing in
Expansion Space A shall remain in the space following the Surrender Date. Tenant
shall deliver to Landlord any plans and specifications, maintenance records,
warranties, permits, approvals and licenses

1

--------------------------------------------------------------------------------

pertaining to the Expansion Space A or to any improvements remaining thereon, or
to both (but not pertaining to Tenant's business conducted therein) in the
possession of Tenant.

      (b) Obligations Until Surrender. Through and including the Surrender Date,
(i) Tenant must continue to pay all Monthly Installment of rent and Additional
Rent as they become due and payable under the Existing Lease, and (ii) all of
the terms, covenants, agreements and conditions of the Lease remain in full
force and effect with respect to the Expansion Space A. As of 11:59 p.m. on the
Surrender Date, the surrender of the Expansion Space A will be deemed effective
and the monetary obligations with respect to the Surrender Space must be
prorated, billed and payable in the manner provided in the Existing Lease, in
the same manner as would apply if the term of the Existing Lease expired on the
Surrender Date with respect to the Surrender Space.

      (c) Effect on Existing Lease After Surrender Date. After the Surrender
Date, the Existing Lease shall continue in full force and effect for the Lease
Term upon and subject to all of the terms and provisions of the Existing Lease,
as amended by this Amendment, including, without limitation, the following
modifications of the Existing Lease:

(i) Tenant shall have no right to possession, use or lease of the Expansion
Space A, or any options or other rights with respect to the Expansion Space A,
except that Landlord shall allow Tenant access to Expansion Space A after the
Surrender Date for the following: (v) access and use of the server room and the
small room next to the server room which contains the fire suppression system,
and the relocation of the server room to the Remaining Premises (defined below),
which relocation shall be completed by no later than August 1, 2008; (w) general
access by employees of Tenant through July 11, 2008; and (x) maintenance and
movement of the small switch and T1 line (the location of which is shown on
Exhibit D attached hereto) in the server room which shall be completed no later
than September 18, 2008. Landlord will instruct its contractor to not cut the T1
line until the earlier of (y) the date the T1 line moved or (z) September 19,
2008. Tenant shall notify Landlord of its desired time(s) of entry to movement
of the T1 line and shall submit for Landlord's approval the name(s) of the
contractor(s) who will perform such work. All such access and work described in
this subsection shall be permitted as long as such entry will not interfere with
the timely and orderly construction and completion of the work to be undertaken
by Landlord in Expansion Space A. Tenant shall be responsible for and be
required to reimburse Landlord for its pro rata share of the electrical costs
incurred in Expansion Space A through and until the completion of such
relocation of the server room within ten (10) days of demand therefor. Such
entry shall be without payment of Monthly Installments of rent or Rent
Adjustments, but such entry and all acts and omissions in connection with it are
subject to and governed by all other provisions of the Existing Lease, including
Tenant's indemnification obligations, insurance obligations, obligations under
Sections 6, 13 and 14;

(ii) the regular Monthly Installment of rent and Additional Rent allocable to
the Expansion Space A shall no longer accrue; and

(iii) the rentable area of the Existing Premises is reduced by 32,256 RSF. The
premises after the Surrender Date is conclusively presumed to be 32,520 RSF (the
"Remaining Premises") located at 995 E. Arques Avenue.

      (d) Holding Over.  In the event that Tenant fails timely to vacate and
deliver exclusive possession of the Expansion Space A to Landlord by the
Surrender Date as required under this Amendment and the Existing Lease, then:

(i) Such holding over shall not constitute a renewal, extension, a month to
month tenancy or other permitted tenancy, but such continued possession shall be
subject to the other covenants, conditions and agreements of the Lease, except
as otherwise expressly provided in this Amendment. Tenant shall be deemed to be
holding over with respect to the Expansion Space A without the express written
consent of Landlord and shall be liable to Landlord for rent with respect to the
Expansion Space A at the holdover rate provided in the Lease and shall indemnify
Landlord against loss or liability resulting from any delay of Tenant in not
surrendering such premises on the Surrender Date, including, but not limited to,
additional costs or losses incurred

2

--------------------------------------------------------------------------------

by Landlord in preparing such space for third parties to whom it is now or
hereafter leased, any amounts required to be paid to third parties who were to
have occupied all or part of such premises, Landlord's loss of income and profit
if payment of rent is deferred or delayed under any lease to a third party, or
if any such lease is cancelled or lost, due to Tenant's failure timely to vacate
and deliver possession, and any attorneys' fees related to the foregoing.

(ii) Such failure shall constitute an event of default and breach of the Lease
without any applicable grace period, notice from Landlord or period to cure; and

      (e) No Release. Notwithstanding any provision of the foregoing to the
contrary, neither this Amendment nor the acceptance by Landlord of the Expansion
Space A shall in any way:

(i) be deemed to excuse or release Tenant from any obligation or liability with
respect thereto (including, without limitation, any obligation or liability
under provisions of the Lease to indemnify, defend and hold harmless Landlord or
other parties, or with respect to any breach or breaches of the Lease) which
obligation or liability (i) first arises on or prior to the date on which Tenant
delivers to Landlord possession of the Expansion Space A, or (ii) arises out of
or is incurred in connection with events or other matters which took place on or
prior to such date, or

(ii) affect any obligation under the Lease which by its terms is to survive the
expiration or sooner termination of the Lease.

      Section 4. Extension of Term; Monthly Installments; Tenant's Pro Rata
Share.

      (a) Landlord and Tenant acknowledge and agree that, before giving effect
to this Amendment, pursuant to the Existing Lease, the Expiration Date of the
Lease Term of the Existing Lease is scheduled to be December 31, 2009.
Notwithstanding any provision of the Existing Lease to the contrary, the
Existing Lease is hereby amended to provide that the Lease Term of this Lease
with respect to the Remaining Premises shall continue for a term of twenty-four
(24) months (the "Third Extended Term") beginning on January 1, 2010 (the "Third
Extended Term Commencement Date") and expiring on December 31, 2011 (hereafter,
the "Expiration Date" with respect to the Third Extended Term), unless sooner
terminated pursuant to the terms of the Lease. Landlord and Tenant acknowledge
and agree that this Amendment provides all rights and obligations of the parties
with respect to extension of the current Term, whether or not in accordance with
any other provisions, if any, of the Existing Lease regarding renewal or
extension, and any such provisions, options or rights for renewal or extension
provided in the Existing Lease are hereby deleted as of the Execution Date
except for the Right of Negotiation set forth in Section 2(g) of the Third
Amendment. The defined term "Lease Term" in the Existing Lease shall be deemed
to include the Third Extended Term.

      (b) Monthly Installment of rent for Third Extended Term and Modification
to Monthly Installment of rent for current Term. Notwithstanding any provision
of the Existing Lease to the contrary, commencing on July 1, 2008 and continuing
through the Expiration Date of the Third Extended Term:

(i) the amount of Monthly Installments of rent due and payable monthly by Tenant
for the Remaining Premises shall be as follows:

Period from/through

Monthly Rate