The signal transducer and activator of transcription 5 (STAT5) is a member of the STAT family of proteins, implicated in cell growth and differentiation. STAT5-IN-1 inhibits STAT5 by binding to the SH2 domain. The functions of the SH2 domains of STAT3, STAT1, and of the tyrosine kinase Lck are inhibited to a lesser extent (IC50>500 µM). STAT5-IN-1 block STAT5/STAT5 DNA binding in K562 nuclear extracts. Substitution of the hydrogen at C6 of the chromone ring by an ethyl group does not affect activity of STAT5-IN-1 against STAT5β, but leads to complete loss of selectivity against other STAT family members.