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
The transcriptionfactorSTAT5b is an antitumor target. Recently, we presented the small molecules Stafib-1 and Stafib-2 as potent, selectiveinhibitors of the STAT5b SH2 domain. Here we report that halogen substitutions on the terminal phenyl ring of Stafib-1 and a close derivative are tolerated and specificity over the STAT5a SH2 domain is maintained, albeit with a slight reduction in activity. Our