USP21 and cancer: BAY-805(21) surpasses the stringent criteria for chemical probestypically applied by the SGC48 and others,including the availability of its less active enantiomer BAY-728 (22) as a corresponding negative control.46,49,50 Both compounds will be available upon request51 and are recommended to be used in cellular studiesto interrogate the function of USP21 in normal physiology and multiplediseases, from viral infections to cancers.52