The covalent grip: A new class of 5‐bromobarbiturates (Cyplecksins; see structure) act by a covalent mechanism to inhibit the biological function of the pleckstrinhomologydomain of cytohesins, small guanine nucleotide exchange factors for the Ras‐like ARF‐GTPases. In cells, Cyplecksins interfere with the phosphoinositol‐dependent membrane recruitment of cytohesins. Cyplecksins may be useful in validating