Insertion of a PGK neomycin resistance cassette into an exon disrupted the Rbl2 gene. Northern analysis of homozygous mutant embryos (E14) using a full length Rbl2 cDNA probe did not detect Rbl2 mRNA. Immunoblot analysis using antisera reactive with the carboxyl-terminal 20 amino acids of RBL2 did not detect RBL2 in homozygous mutant embryos.