When toxicologists warned that the plastics ingredient BPA might be harmful, consumers clamored for something new. But problems persist. The similarity of BPS's structure to that of BPA is enough to raise suspicions that it may mimic estrogens, says Cheryl Watson, a biochemist at UTMB. Natural estrogens are small molecules containing several phenolic rings; these bear chemical adornments that bind to a pocket found in estrogen receptors in the body. BPA and BPS are about the same size and have similar phenolic rings with similar attachments, so they may slot like keys into estrogen receptors, Watson says. Watson and a colleague, Rene Viñas, now at the U.S. Food and Drug Administration, measured the responses of cultured rat pituitary cells to BPS.