Table 2.

Genes expressed in SCs	Genes expressed in TACs	Organ or tissue	Putative TAC function	References
GFRα1+	GFRα1−/Miwi2+/Ngn3+	Testis	Represent a novel subpopulation of undifferentiated spermatogonium. Also involved in TAC pluripotency	[24]
β1 integrinsHi/α6 integrinHi/CD71Low/delta1Hi/desmoglein 3Low/EGFR1Low/Lrig1Hi MCSP+/delta1+	β1 integrinsLow MCSP−/delta1−	Epidermal cells	May represent a new population without any characterized function	[88–90]
Reg4	Reg4/ribosomal genes	Intestinal epithelium	TAC populations migrating upward along the intestinal crypt-villus axis	[87]
β1 integrinHi/keratin14 p63 Hi/Pp63Low	β1 integrinLow/p63 Low/Pp63Hi	Clonogenic cultures and keratinocytes cultures	Increased p63 phosphorylation marks the exit from SC state and could be used to detect epidermal cell stratification	[16, 18]
Gas1+ in Bu-SCs/SHH−	Gas1−/SHH+	Hair follicle in mammals	TACs act as a signaling center between Bu-SCs and DP promoting their proliferation. TACs integrate the timing and frequency for two populations of SCs	[59]
p63/PCNA in basal cells	p63/PCNA in suprabasal cells	Zebrafish epidermis	A proliferation shift from basal to suprabasal cells marks the stratification process	[46]
Jak-STAT signaling		Drosophila spermatogonium	The ability of TACs to respond to signals from the SC niche and dedifferentiate into SCs	[22]
Symmetric cell division (SCD) MINSC-PAR3- and LGN- uncoupling of NuMA	Asymmetric cell division (ACD) MINSCHi, PAR3Hi, LGNHi in complex with NuMA	Epidermis stratification	Asymmetric division is essential for TACs formation	[33–39]
p21Hi/p27Hi/RB dephosphorylated (active form)	CDK2Hi CDK4Hi/p21Low/RB phosphorylatedDown (inactive form)	Mouse pituitary gland and hematopoietic cells	Events that may trigger TAC cell proliferation	[49–51]