The entire coding region was replaced with an in-frame cDNA for mouse Thrb2 and a floxed neo cassette via homologous recombination. Cre mediated recombination removed the neo cassette. Western blot analysis indicated protracted expression of Thrb2 and absence of expression of Nrl in mice at E17.5. In situ hybridization showed ectopic expression of Thrb mRNA over the outer neuroblastic layer of the retina at P7 in homozygous and heterozygous mice.