The major and minor beta chains were replaced with a construct made of a 5.66 kb fragment of the human gamma chain including 1400 bp of 5' flanking sequence and a 4.1 kb fragment of the sickle cell form of human HBB in ES cells already containing a replacement of Hba complex with human HBA, Hbatm1(HBA)Tow. Cre-mediated recombination removed the floxed hygro cassette. (J:44160, J:134980)