The profiles of both compounds 1 (Figure ) and 2 (Figure ) were consistent with competitive inhibition, displaying an increased Km for ATP (3.6-fold from 3.7 µM to 13.2 µM and 4.7-fold from 3.7 µM to 17.3 µM respectively) with no significant effect on kcat (9.2 vs 6.6 µM/min and 9.2 vs 6.5 µM/min respectively). A jump dilution experiment did not result in a shifted IC50, suggesting reversible binding. Most unexpectedly, a UV-crosslinking experiment with [ 33 P]-labeled ATP showed selectivity for the MSH3 subunit (unique to MutSβ) over the MSH2 subunit (common to both MutSβ and MutSα). As inhibition of MutSα is known to promote tumor formation, selectivity between these two isoforms is of particular importance.