(B) Top gel: PCR genotyping of Atrx alleles in embryos using primers PPS1.15 (exon 17) and Mxnp30 (exon 20) as described in Protocol S1. The sizes of PCR products from the different alleles are indicated. Both the Atrx Δ18 (resulting from recombination event B in Figure 2A) and the Atrx Δ18Δneo allele (resulting from recombination event C in Figure 2A) are null for full-length Atrx protein. The bottom gel shows products from a PCR reaction (primers DG52/DG53) used to sex embryos as described in Protocol S1. A 450-bp PCR product is amplified from a mouse Y chromosome-specific satellite repeat.