Fig. 1. Diagram of modified Ascl1 BACs and fate-mapping strategy. The Ascl1 gene is located on mouse chromosome 10. The BAC RPCI 428P21 encompasses the Ascl1 coding sequence with 100 kb 5' and 200 kb 3' (orientation 5' to 3' relative to Ascl1). The genes encoding Pah4 (phenylalanine 4-hydroxylase) and Pah (phenylalanine hydroxylase) are 30 kb and 78 kb 5' of Ascl1, and are included in the BAC RPCI 428P21. Ascl1-GIC and Ascl1-CreER^TM are modifications of RPCI 428P21 that replace the Ascl1 coding sequence with that encoding GFP and Cre separated by an IRES, or a tamoxifen-inducible Cre (Hayashi and McMahon, 2002) with a heterologous 3' polyA addition cassette from bovine growth hormone, respectively. The R26R-reporter mice were previously described (Soriano, 1999; Srinivas et al., 2001). Protein from these reporters is only made after the STOP sequence is recombined out by Cre recombinase activity.