Signal transducers and activators of transcription (STATs) are a family of transcription factors. STAT5A and 5B are two highly related proteins encoded by two distinct genes. Transgenic knockout mice studies have indicated the importance of STAT5 proteins for the regulation of both lactation and growth performance. Moreover, different studies determine the role of STAT5 proteins in the modulation of adipocyte function. In this study, we sequenced one fragment of STAT5B gene from animals of six breeds (Duroc, Iberian, Landrace, Large White, Piétrain and Meishan) to identify genetic variants. A G/A single nucleotide polymorphism in intron 14 creates a polymorphic PstI restriction site and was genotyped by polymerase chain reaction restriction fragment length polymorphism in the six breeds. Allele G was only present in Large White, Piétrain and Meishan populations, detecting only G allele in this last pig breed. The STAT5B gene was located by radiation hybrid mapping to porcine chromosome 12, within the confidence interval for the fatty acid composition quantitative trait loci, previously identified in an Iberian x Landrace cross. © 2006 The Authors.