SOX2 and neoplasm: A number of studies have isolated cells within human and murine PAs (Xu et al. 2009, Hosoyama et al. 2010, Chen et al. 2014, Donangelo et al. 2014, Mertens et al. 2015), fulfilling some or all of the CSC criteria as follows: (1) clonogenic ability in vitro, (2) stem cell marker expression (e.g. CD133, CD166, SOX2, SCA1), (3) multipotency, (4) resistance to chemotherapeutic agents, (5) ability to initiate de novo tumours when transplanted into immunocompromised hosts (Welte et al. 2010, Martinez-Barbera & Andoniadou 2016).