PROM1 and pachyonychia congenita: Importantly, accumulating lines of experimental evidence have indicated that PC- and metastasis-initiating cells expressing stem cell-like markers such as CD133+, CD44high, ALDHhigh, ABCG2+ and/or CXCR4high and endowed with a high self-renewal ability can play critical functions for PC progression, metastases and resistance to current clinical therapies [45, 53, 95, 97, 191–194].