Generation of Alk1-conditional alleles. (A) Schematic diagram of the Alk1 wild-type allele, Alk1-conditional targeting vector, and Alk1^3loxP, Alk1^2loxP, and Alk1^1loxP alleles. Exons and loxP sequences are indicated by boxes and arrowheads, respectively. Locations of primer pairs used for amplifying specific regions containing a loxP sequence are also indicated. (B) Genomic Southern blot analysis from EcoRI digested DNA isolated from several ES clones, showing the homologous recombination of the Alk1^3loxP vector into the Alk1 locus. (C) Representative PCR genotyping results from intercrosses of Alk1^+/3loxP (top) and Alk1^+/2loxP (bottom). The arrowheads indicate the PCR amplicon containing the loxP sequence; Alk1^3loxP/3loxP (lanes 2 and 5); Alk1^+/3loxP (lanes 1, 4); Alk1^+/+ (lane 3); Alk1^+/2loxP (lanes 6 and 8); and Alk1^2loxP/2loxP (lanes 7, 9, and 10). (D) PCR detection of the Alk1^1loxP allele from genomic DNA isolated from multiple organs/tissues of E16.5 L1cre(+)Alk1^3loxP/3loxP (top), L1cre(+)Alk1^+/3loxP (middle), and L1cre(−);Alk1^3loxP/3loxP (bottom) fetuses, demonstrating tissue-specific Cre activities. The arrowheads indicate the Alk1^1loxP-specific PCR amplicon.