A 2.9 kb fragment of the 5' flanking sequence and the 83-bp first intron of the human CD68 gene was cloned into a vector in front of codon-improved cre and polyadenylation sequences. 2 founder lines were selected for analysis, based on expression levels in isolated peritoneal F4/80+ cells. One line expressed cre in sperm and egg cells so the other line was used for further studies.