The targeting vector is designed with (from 5' to 3') a CMV-IE enhancer/chicken beta-actin/rabbit beta-globin hybrid promoter (CAG), a loxP-flanked STOP cassette, a FRT-flanked STOP cassette, an H2B-EGFP fusion protein (coding sequence for the human HIST1H2BB gene [histone 1 H2bb] followed C-terminally by an enhanced green fluorescent protein sequence), and a Neo-polyA cassette. The entire construct was inserted between exons 1 and 2 of the Gt(ROSA)26Sor locus.