A 7.8 kb genomic fragment within exon 1 and intron 1 was replaced by a neomycin selection cassette in opposite transcriptional orientation to the gene via homologous recombination, creating a stop codon after 43 aa. Western blot with an antibody against the COOH terminus and immunofluorescence analysis confirmed the absence of protein in E10.75 embryos and cardiac tissue, respectively.