Dr. Clara M. Szego passed away recently at the age of 101. She was a pioneer in the study of steroid hormone action, particularly the role of estrogens in target tissues. She was first to report on steroid hormone binding with proteins in the circulation in 1946. This observation led to a Ciba Foundation Award of the Endocrine Society in 1953. Claire then followed the steroid hormone and reported on interactions with membrane-associated and nuclear proteins in target cells of the reproductive tract and breast. She was also an advocate to stop the use of diethylstilbestrol (DES) in animal feed in the 1950s–1960s and later helped to develop clinical tests to identify women at risk for developing malignancy after exposure to DES in utero. Being a woman in endocrinology was challenging in the post-war period, and Claire worked to hinder gender bias in academia throughout her career. Her impact on the fields of endocrinology and steroid hormone action was profound, and the full ramifications and clinical benefits of her work remain to be realized.