Mammalian spermatogonial stem cells (SSCs), the resident testicular germline stem cell population, are derived from postnatal quiescent progenitor cells (licensed T2-prospermatogonia). Within the adult testis, SSCs have the unique ability to self-renew or divide into more differentiated progeny (Kluin & de Rooij 1981, Yoshida et al. 2006). SSCs are definitively defined by their stem-like qualities (proliferation, self-renewal and expansion) using functional assays, such as in vivo transplantation, in vitro clonal proliferation and in vitro cobblestone assays; it has proven difficult to determine SSCs precisely in vivo with a single molecular marker, because many of their phenotypic characteristics overlap with their progeny, the A-type undifferentiated spermatogonia (Ploemacher et al. 1989, Brinster & Zimmermann 1994, Dobrinski et al. 1999, Kubot a & Brinster 2006, Kanatsu-Shinohara et al. 2012). Classically, SSCs are characterized in many mammalian systems by the expression of a combination of markers, such as cadherin 1 (CDH1), glial cell line-derived neurotrophic factor family receptor alpha 1 (GFRA1), inhibitor of differentiation 4 (ID4), integrins alpha 6 and beta 1, strong expression of zinc finger and BTB domain containing 16 (ZBTB16HI; also known as promyelocytic leukemia zinc finger (PLZFHI)), ret proto-oncogene (RET) and Thy1, and the lack of kit oncogene (KIT) and stimulated by retinoic acid 8 (STRA8) expression