Seattle Genetics, the region’s biggest biotechnology company by market capitalization, said chief medical officer Thomas Reynolds is resigning effective Dec. 31 “in connection with his retirement for a family health matter.”

Reynolds, who turned 53 in March, joined the company in 2007 and led its clinical and regulatory teams during the development and approval of its cancer drug Adcetris.

The company said it has started the search for his replacement.

Its stock price closed Friday at $24.33 a share, down 4 percent.