Targeting an endogenous murine locus using homologous recombination. Coding exons of the targeted locus are represented as solid boxes. The cDNA of interest and PGK-neo selectable marker cassette are represented as open boxes. Transcriptional start sites for the targeted gene locus and PGK-neo selectable marker cassette are indicated by arrows. The 5′ untranslated region is represented by a hatched box. LoxP sites used for CRE recombinase-mediated selection cassette excision are represented as open ovals.