Androgen signalingin prostate cancer cells involvesmultisitecysteine ADP-ribosylation of the androgen receptor (AR) by PARP7.The AR modification is read by ADP-ribosyl binding macrodomains inPARP9, but the reason that multiple cysteines are modified is unknown.Here, we use synthetic peptides to show that dual ADP-ribosylationof closely spaced cysteines mediates recognition by the DTX3L/PARP9complex. The gene discussed is TIPARP; the disease is prostate cancer.