EXHIBIT 4.3

FORM OF LOCK-UP AGREEMENT


September 9, 2009

Geron Corporation
230 Constitution
Drive
Menlo Park, California 94025
Attention: Chief Financial Officer


Ladies and Gentlemen:

     Geron
Corporation, a Delaware corporation, (the “Company”), and ________ (the
“Investor”) have entered into a Common Stock and Warrant Purchase Agreement
dated as of the date hereof (the “Purchase Agreement”), providing for the
purchase of _______ shares (the “Shares”) of the Company’s common stock, par
value $0.001 (“Company Common Stock”) and a warrant (the “Warrant”) to purchase
_______ shares of Company Common Stock (the “Warrant Shares”) by the Investor.
The Shares, the Warrant and the Warrant Shares are collectively referred to
herein as the “Securities.”

     The Investor
hereby agrees that, without the prior written consent of the Company, it will
not, during the period commencing on the date hereof and ending on September 9,
2010 (such period referred to herein as the “Lock-Up Period”), offer, sell,
contract to sell, pledge or otherwise dispose of, directly or indirectly, any of
the Securities or securities convertible into or exchangeable or exercisable for
any shares of Securities, enter into a transaction which would have the same
effect, or enter into any swap, hedge or other arrangement that transfers, in
whole or in part, any of the economic consequences of ownership of the
Securities or such other securities, in cash or otherwise, or publicly disclose
the intention to make any such offer, sale, pledge or disposition, or to enter
into any such transaction, swap, hedge or other arrangement (collectively, the
foregoing are referred to herein as “Trading Activity”). The restrictions on
Trading Activity during the Lock-Up Period, as set forth herein, are referred to
as the “Lock-Up”. Notwithstanding anything to the contrary set forth herein, the
Company agrees that the Lock-Up shall be waived, and the Investor may engage in
Trading Activity, at any time during the Lock-Up Period in which each of the following
requirements are contemporaneously met: (a) the closing prices of the Company
Common Stock (as adjusted for stock splits, dividends, combinations and similar
events), as quoted on The Nasdaq Global Market (“Nasdaq”), for each of the five
(5) consecutive trading days immediately preceding such Trading Activity are
greater than or equal to $15.00 per share, and (b) the Trading Activity is
executed at a price, or prices, as quoted on Nasdaq, greater than or equal to
$15.00 per share. The provisions of this paragraph shall not apply to
transactions relating to shares of Company Common Stock or other securities in
each case acquired in open market transactions.

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     The Investor
confirms that this Lock-Up Agreement is irrevocable and shall be binding on the
Investor’s legal representatives, successors and assigns. The Investor agrees
and consents to the entry of stop transfer instructions with the Company’s
transfer agent against the transfer of the Securities except in compliance with
the terms and conditions of this Lock-up Agreement. This Lock-up Agreement shall
be governed by, and construed in accordance with, the laws of the State of
California.


	
      Very truly yours,

	 
	
      [Investor]

	 
	 	
	By: 
    	 
		Name:
  
		Title:
  




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