The Spen protein family is found in worms, flies, and mammals, and is implicated in diverse biological processes from embryogenesis to aging. Spen proteins have three N‐terminal RNA recognition motifs and a C‐terminal SPOC domain. The mammalian Spen proteins Mint and its human ortholog SHARP interact with the Notch‐signaling mediator RBP‐J as well as Msx2 and several unliganded nuclear hormone receptors, and impart transcription‐repressing activity to these molecules by recruiting corepressors through the SPOC domain. Despite these in vitro findings, Mint/SHARP's physiological role is largely unknown, because Mint germline knockouts are embryonic lethal. To analyze Mint/SHARP function in postnatal mice, we created Mint‐floxed mice that allow the Cre/loxP‐mediated conditional knockout of Mint. We analyzed Mint and RBP‐J epistasis during Notch‐dependent splenic B‐lymphocyte development, and found that Mint suppresses Notch signaling through RBP‐J. In addition, Mint deficiency caused severe hypoplasia in postnatal brain, suggesting it may regulate neuronal cell survival. genesis 45:300–306, 2007. © 2007 Wiley‐Liss, Inc.