NEW YORK — Biotechnology company BioTime Inc. on Thursday proposed two transactions that would combine its stem cell therapy assets with those of Geron Corp.

In a letter to Geron shareholders, BioTime suggests two deals that would form a new publicly traded company owning Geron's stem cell assets with some of BioTime's. It said Geron could have a stake of up to 45 percent in the company, which would also hold $40 million in BioTime stock along with shares of some BioTime stem cell subsidiaries. Geron shareholders would also receive BioTime stock-purchase warrants now valued at about $13 million.

Geron, of Menlo Park, Calif., was a pioneering developer of stem cell therapies. But in November 2011 it said it would exit that business and focus on development of other products, including its experimental cancer drug imetelstat.

"Nearly a year has now passed, and Geron has not announced any transaction that would provide value to either Geron or its shareholders from those stem cell assets," BioTime said in a press release. The Alameda, Calif., company said Geron shareholders should contact the board of directors and urge the company to have discussions with BioTime.

Geron did not immediately respond to a request for comment.

BioTime was founded by former Geron scientists, and when Geron said it was ending its stem cell business, analysts said BioTime was one of the companies that might be interested in buying its treatments.

Geron shares rose 12 cents, or 8.5 percent, to $1.54 in afternoon trading. Shares of BioTime lost 4 cents to $3.95.

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