TAT1 is known to associate with the transcription factors p48, Sp1, and p300. STAT1 is activated by peptide hormones, growth factors and cytokines, particularly cytokines of the interferon family. STAT1 alpha and STAT1 beta isoforms arise by alternative splicing of a single gene. The only difference between the two proteins is that STAT1 beta lacks 38 carboxy terminal amino acids. Contained within these 38 terminal amino acids of STAT1 alpha is a critical serine residue (Ser 727) whose phosphorylation is required for maximal transcription activity induced by IFN gamma.