Using PCR-heteroduplex and sequencing we identified A/G substitution at position -488 in the promoter region of the bovine STAT5A gene. With reverse transcription-PCR (RT-PCR) and other means we showed that the STAT5A expression level in the liver was higher in cattle with AA than GG genotype. Electrophoretic mobility shift assay (EMSA) showed that the A-->G transition at position -488 increased the STAT5A gene promoter binding capacity for liver nuclear proteins [possibly hepatic nuclear factor (HNF)-3]. Copyright 2004 Elsevier B.V.