This transgene consists of the 1.3-kb murine Rb1 promoter region, the first exon and intron fused, in frame, to exons 2 to 27 of the mouse Rb1 cDNA and a human growth hormone cassette that provides splicing and polyadenylation signals at the 3' end. LoxP sites were inserted into the first intron and between the Rb1 stop codon and the human growth hormone, in a tandem orientation, to allow excision in the presence of Cre recombinase. Only low levels of protein were expressed, demonstrating that this transgene is hypomorphic. Six founders were generated and line 1 was selected as the representative line.