The Cre recombinase coding sequence, modified by addition of a Kozak consensus translation start sequence and nuclear localization signals, was inserted into the p1017 expression vector between the thymocyte-specific, mouse proximal Lck promoter and most of the human growth hormone gene, extending from within exon 1 through the polyadenylation signal. Cre recombinase activity is restricted to thymocytes; evidence of activity in spleen is attributed to the presence of thymus-derived T lymphocytes. (J:67674, J:81268)