Abstract ERα17p is a peptide corresponding to the sequence P295LMIKRSKKNSLALSLT311 of the estrogen receptor alpha (ERα) and initially found to interfere with ERα-related calmodulin binding. ERα17p was subsequently found to elicit estrogenic responses in E2-deprived ERα-positive breast cancer cells, increasing proliferation and ERE-dependent gene transcription. Surprisingly, in E2-supplemented media, ERα17p-induced apoptosis and modified the actin network, influencing cell motility. Here, we report that ERα17p internalizes in breast cancer cells (T47D, MDA-MB-231, SKBR3) and induces a massive early (3 h) transcriptional activity. Remarkably, about 75% of significantly modified transcripts were also modified by E2, confirming the pro-estrogenic profile of ERα17p. The different ER spectra of the used cell lines allowed us to identify a specific ERα17p signature related to ERα as well as its variant ERα36. With respect to ERα, the peptide activates nuclear (cell cycle, cell proliferation, nucleic acid and protein synthesis) and extranuclear signaling pathways. In contrast, through ERα36, it mainly triggers inhibitory actions on inflammation. This is the first work reporting a detailed ERα36-specific transcriptional signature. In addition, we report that ERα17p-induced transcripts related to apoptosis and actin modifying effects of the peptide are independent from its estrogen receptor(s)-related actions. We discuss our findings in view of the potential use of ERα17p as a selective peptidomimetic estrogen receptor modulator (PERM).