A Dll1 minigene (exons 1 to 9 as cDNA followed by the remainder of the Delta1 gene intron 9, exon 10, intron 10, and exon 11) with nucleotide substitutions that result in the amino acid substitutions of valine for threonine at postition 638 (T638V) and alanine for serine at positions 693 (S693A) and and 696 (S696A) and a floxed neomycin resistance cassette were inserted downstream of exon 1. Cre-mediated recombination removed the selection cassette.