The entire gene was replaced with a fusion of the viral coactivator VP16 activation domain and Rbpj, a floxed neo cassette, and a poly A tail. Cre mediated recombination removed the neo cassette. This allele expresses a constitutively active form of Rbpj. Whole mount in situ hybridization analysis confirmed the absence of Mesp2 expression in E11.5 homozygous embryos.