The transgene comprises a yeast artificial chromosome (YAC) from the CEPH Whole Genome MegaYAC Library carrying a 475-kb fragment of DNA from human Chromosome 21 containing the genes HLCS (holocarboxylase synthetase (biotin-(proprionyl-CoA-carboxylase (ATP-hydrolysing)) ligase)), PIGP (phosphatidylinositol glycan anchor biosynthesis, class P; formerly DSCR5), TTC3 (tetratricopeptide repeat domain 3) and DSCR3 (Down syndrome critical region gene 3). This transgenic line was shown by quantitative PCR analysis to bear three copies of the YAC.