Geron Corp., which develops treatments for cancer and other diseases, said Monday it plans to sell an undisclosed amount of stock to the public.

The shares fell sharply in extended trading.

Geron said the plan was subject to market conditions and it couldn't guarantee the size or terms of the offering, other than saying it would give underwriters the option to buy another 15 percent of shares to cover overallotments.

J.P. Morgan Securities LLC and Lazard Capital Markets LLC are underwriters.

Geron is working with human embryonic stem cells to develop treatments for cancer and other conditions. In July, the U.S. Food and Drug Administration gave the company permission to conduct a clinical trial using embryonic stem cells to treat spinal cord injury.

In October, the company reported a loss of $18.3 million on revenue of $546,000.

On Monday, the shares gained 11 cents to close at $6.12 in regular trading before Geron disclosed the proposed sale of stock. In after-hours trading, they fell 46 cents, or 7.5 percent, to $5.64. They have ranged from $4.37 to $6.67 in the past year.