A 310 bp fragment 5' of the initiation codon of the rat growth hormone gene (Gh1) was used to drive cre recombinase in the construct. The purified construct was injected into FVB zygotes. Line b was determined (by Southern blot) to carry 3 copies of the insertion was established and analyzed in detail. RT-PCR results demonstrated high levels of cre mRNA expression in the pituitary, lower levels in the testis, and no expression in any other tissues. (Cre expression was also detected in cephalic extracts from animals at E17.)