Fig. 1. Constructing Wnt3a-IRES-Cre mouse lines to fate map the hem. (A, top to bottom) Wild-type allele of Wnt3a; the targeting vector containing an IRES-Cre cassette; the targeted Wnt3a-IRES-Cre allele with Cre inserted into the 3' end of the Wnt3a locus. (B,C) Coronal sections through E12.5 forebrains show Cre expression in the hem of Wnt3a-IRES-Cre mice (C), indistinguishable from expression of endogenous Wnt3a (B).