Progestin drives breast cancer growth by inducing p21CIP1 expression through the assembly of a transcriptional complex among Stat3, progesterone receptor and ErbB-2

► Stat3, PR and ErbB-2 are necessary for progestin-induced upregulation of p21CIP1. ► Progestin recruits Stat3, PR and ErbB-2 to a Stat3 binding site on p21CIP1 promoter. ► ErbB-2 acts as a coactivator of Stat3 in progestin-induced p21CIP1 transcription. ► p21CIP1 is essential for progestin-induced breast cancer growth.