Kid, the toxin of theparD(kis, kid) maintenance system of plasmid R1, is an endoribonuclease that preferentially cleaves RNA at the 5¢ of A in the core sequence 5¢-UA(A⁄C)-3¢. A model of the Kid toxin interacting with the uncleavable mimetic 5¢-AdUACA-3¢ is available.