UNC3866 is a potent antagonist of the methyllysine (Kme) reading function of the Polycomb CBX and CDY families of chromodomains. UNC3866 binds the chromodomains of CBX4 and CBX7 most potently, with a K(d) of ∼100 nM for each, and is 6- to 18-fold selective as compared to seven other CBX and CDY chromodomains.