Steroid hormone receptors are a class of the nuclear receptor family whose mechanism of transcriptional regulation has been under intensive study for over 30 years. Yet beginning even 30 years ago there were suggestions that at least some effects of steroids, particularly proliferative and electrophysiological effects, might be mediated at the plasma membrane (refs. 1–3 for review). The evidence was based largely on immunofluorescence and the rapidity of the effects, but the entire field of nongenomic signaling by nuclear receptors was not widely appreciated until the recent demonstration that both the estrogen receptor and the progesterone receptor (PR) can activate the cytoplasmic tyrosine kinase c-Src and the mitogen-activated protein kinase (MAPK) pathway (4, 5). Very recently this activation of c-Src has been shown to be associated with binding of a prolinerich sequence in PR to the c-Src Src homology 3 domain, leading to reduced autoinhibitory repression of c-Src kinase activity (V. Boonyaratanakornkit, M. Porter-Scott, V. Ribon, L. Sherman, SM Anderson, TW Miller, and DP Edwards, personal communication). The activated c-SrcPR complex then mediates activation of the MAPK pathway. The proline-rich sequence of PR also binds to the Src homology 3 domain of Cblassociated proteins and other signaling molecules (Boonyaratanakornkit et al., personal communication). c-Src is acylated at its N terminus with myristic acid, a modification thought to direct its localization to the plasma membrane (7). Thus, PR activation of c-Src is a presumed plasma membrane effect of PR. However, active c-Src also has been localized on the nuclear periphery (8), making it unclear where PR activation of c-Src occurs.