Replacement of the murine adult β globin genes by the human βIVS-2-654 gene. The socket-containing chromosome (A), the th-4 plug targeting construct (B), and the correctly targeted chromosome (C) are shown. The exons and introns of genes are represented as boxes and thick lines, respectively. The human β globin gene is cross-hatched with the position of the IVS-2-654 mutation shown with an asterisk. Promoter (P) and exons 1-9 of HPRT are marked. Upstream and downstream sequences that are identical or homologous in the targeting construct and the target chromosome are demarcated by dashed lines. βh3 is a β globin pseudogene. Recombination (indicated by Xs) occurs between the target locus (A) and the plug targeting construct (B), yielding a chromosome that contains the human β globin gene in place of the adult murine β globin genes, β major and β minor. Additionally, the neo gene is removed and a functionalHPRT gene is created by the correct targeting. TheHPRT gene and globin genes are transcribed from left to right in the figure, the neo gene is transcribed from right to left.