A 5.7 kb human A gamma globin (HBG1) gene fragment (GenBank id: U01317: 38,066-43,728), a 4.1 kb human beta globin (HBB) gene fragment, and a floxed pgk-hygromycin cassette replaced the Hbb-b1 and Hbb-b2. The human beta globin gene fragment contained a single G to A mutation in the first base of intron 1 destroying the normal splice donor site. The switch from expression of HBG1 to expression of HBB is delayed until after birth. The mutant HBB locus produces only short lived aberrantly spliced transcripts. (J:147906)