more than 50‐fold selectivityoverSTAT5a. The binding site of the core moiety of Stafib‐1 was validated by functional analysis of point mutants. A prodrug of Stafib‐1 was shown to inhibitSTAT5b with highselectivityoverSTAT5a in tumor cells. Stafib‐1 provides the first demonstration that naturally occurring SH2 domains with more than 90 % sequence identity can be selectively targeted with small