Gene targeting ofLyl-1. Structure of the gene targeting vector (A) and the genomic locus of Lyl-1 before (B) and after (C) homologous recombination. A 0.8-kb Hpa I fragment of the Lyl-1 gene, encoding the bHLH domain, was replaced by a LacZ/Neo gene cassette allowing expression of a Lyl-1^LacZ fusion gene. Relevant restriction enzyme sites and the lengths of diagnostic fragments are shown. Southern blot analysis of homologous recombinant ES clones 4 and 10 and wt E41.1 cells (D) and mice tail DNA (E-F) confirmed the targeted inactivation of the Lyl-1 gene. Probes were as indicated.