Trim33 (Tif1γ, ectodermin, moonshine), a member of the TIF1 family of transcriptional coactivators and corepressors, is a large nuclear protein that contains an N‐terminal tripartite (Trim) domain composed of a RING domain, two B‐box domains, and a coiled coil domain. It has been suggested that Trim33 (Ectodermin) mediates ectodermal induction in the Xenopus by functioning as a Smad4 ubiquitin ligase, while in the zebrafish Trim33 (moonshine) has been reported to act as a R‐Smad binding protein in induction of erythroid differentiation. Since the developmental role of Trim33 in mammals is currently unknown, we generated mice carrying the conditional Trim33 (Trim33^FX) allele by flanking exons 2–4 encoding most of the functionally critical N‐terminal tripartite domain by loxP sites. We confirmed the null genotype by using the EIIa‐Cre transgenic approach to create mice that lack exons 2–4. Embryos deficient in Trim33 die during early somitogenesis, demonstrating that Trim33 plays an important nonredundant role in mammalian embryonic development. genesis 46:329–333, 2008. © 2008 Wiley‐Liss, Inc.