The Rbl1 gene was disrupted by insertion of a PGK neomycin resistance cassette into exon 4 immediately downstream of the sequence encoding amino acid 165. Northern analysis of embryonic fibroblast confluent cultures detected a slightly truncated Rbl1 transcript in homozygous mutant E14.5 embryos. The authors propose that the truncated transcript would be initiated from the PGK promoter. Immunoblot analysis using antisera reactive with the carboxyl-terminal 18 amino acids of RBL1 did not detect protein in homozygous mutant embryos.