In response to specific ligands, various STAT proteins (signal transducers and activators of transcription) are phosphorylated on tyrosine by Jak protein kinases and translocated to the nucleus to direct gene transcription. Selection of a STAT at the interferon gamma receptor as well as specific STAT dimer formation depended on the presence of particular SH2 groups (phosphotyrosine-binding domains), whereas the amino acid sequence Surrounding the phosphorylated tyrosine on the STAT could vary. Thus, SH2 groups in STAT proteins may play crucial roles in specificity at the receptor kinase complex and in subsequent dimerization, whereas the kinases are relatively nonspecific.