The use of large genomic clones for transgenesis is necessary when the gene to be expressed is too large to be accommodated in a plasmid-based vector. Large insert clones can be produced with the P1 bacteriophage (P1) (1), P1 artificial chromosome (PAC) (2), bacterial artificial chromosome (BAC) (3), or yeast artificial chromosome (YAC) (4) cloning systems. Correct tissue, spatial, and developmental expression can only be achieved when all of the endogenous control elements are present in their native context, and this often requires a large genomic construct. This is particularly important for studying the expression of a gene or gene loci regulated by distant DNA elements. The use of large genomic clones to generate transgenic mice also reduces the possibility of positional effects that can be exerted by the chromosomal sequences flanking the point of transgene integration. Recently, large genomic clones have been used in mouse transgene complementation studies to discover new genes (5).