A targeting vector containing a 3.4 kb EGFP/cre cassette with a 3' Frt-flanked neomycin cassette was introduced into the 5' untranslated region of Hand1. Homologous recombination resulted in deletion of the first exon of Hand1, producing a null allele, and insertion of the EGFP/cre gene under control of the endogenous promoter. Following germline transmission, the neo cassette was deleted by crossing mice to FLPeR homozygous mice.