A construct composed of the 9.2 kb Alk1 regulatory fragment (containing a 2.7 kb promoter region and exon1-3 of Alk1), the SV40 splicing donor and acceptor (SD/SA), an internal ribosome-entry site (IRES), a polyadnylation signal and the cre recombinase sequence was used to generate the B1 transgenic cre line, as well as three other founder lines including the L1- and D-lines. B and D
lines showed similar staining patterns when crossed to lacZ reporter animals.