The product of the multiple endocrine neoplasia type 1 (MEN1) tumor suppressor gene, menin, is an integral component of MLL1/MLL2 histone methyltransferase complexes specific for Lys4 of histone H3 (H3K4). We show that menin is a transcriptional coactivator of the nuclear receptors for estrogen and vitamin D. Activation of the endogenous estrogen-responsive TFF1 (pS2) gene results in promoter recruitment of menin and in elevated trimethylation of H3K4. Knockdown of menin reduces both activated TFF1 (pS2) transcription and H3K4 trimethylation. In addition, menin can directly interact with the estrogen receptor-α (ERα) in a hormone-dependent manner. The majority of disease-related MEN1 mutations prevent menin-ERα interaction. Importantly, ERα-interacting mutants are also defective in coactivator function. Our results indicate that menin is a critical link between recruitment of histone methyltransferase complexes and nuclear receptor-mediated transcription.