PRRT2 and neoplasm: Studying the predicted structure–activity relationships, such as those between promising royleanone compounds and different subtypes of PKC and types of cancer cells, such as the hard to treat triple-negative cancer stem cells or tumor-initiating cells, can provide further indications to guide the design of compounds that can inhibit particular PKC subtypes, or all of them, as well as being active against aggressive triple-negative cancer cells.