Expression of the adult β-globin genes in mice heterozygous for targeted deletions. The Hbb^D allele (D) carries the targeted mutation, while the Hbb^S allele (S) is wild-type. Pools of 100 reticulocytes were isolated from peripheral blood by flow cytometry and analyzed by RT-PCR with primers that amplify both adult genes from both alleles. Analysis of 2 representative independent pools is shown for each mouse strain. WT indicates wild-type D/S animals. Other strain designations are described in “Results.” S and D mark the RT-PCR product from the S and D alleles, respectively.