HDM-2 in the membrane, which normal cells do not show

HDM-2 is the human version of MDM-2, the protein that normally keeps p53 in check inside the nucleus. Cancer cells of many kinds also display it in their outer membrane, and untransformed cells largely do not. That difference in location, rather than any difference in chemistry, is the entire basis of selectivity here. PNC-27 carries the exact piece of p53 that grips HDM-2, so it docks on cells that display it and ignores the ones that do not.