Chiron Corp. said co-founder Edward Penhoet will step down as president and CEO following a disappointing year for the biotechnology company. Chiron said it's looking for a replacement to run the company's day-to-day operations and assume some of the duties of Chairman and co-founder William Rutter. Penhoet will become vice chairman. Analysts said Emeryville-based Chiron has had problems closely managing its different businesses, which sell products ranging from vaccines to a drug to treat multiple sclerosis. Among the firm's disappointments in 1996 was its discontinuation of the development of a once-promising genital herpes vaccine that ended up not working. Chiron shares lost 37.5 cents to close at $18.625 in Nasdaq trading.