Sangamo Therapeutics Inc. shares fell in the extended session Tuesday after the biotech company announced a secondary offering of its stock. Sangamo shares fell 8.5% after hours, following a 1.9% decline to close at $17.80 in the regular session. The company said it planned to offer $200 million in shares, with an additional $30 million to cover overallotments. At Tuesday's closing price the offering and overallotment works out to around 12.9 million shares. Sangamo has about 86 million shares outstanding. Sangamo listed BofA Merrill Lynch, J.P. Morgan Securities, and Cowen and Co. as book runners for the offering.