Table 1.

Overview over pioneering mechanisms.

Pioneer factor	Cellular context of pioneering	Proposed pioneering mechanism	References
Ascl1	- Direct lineage conversion from fibroblasts or other somatic cells to neurons - Neuronal differentiation from ESCs	Nucleosomal targets contain an extra “G” nucleotide at the 3′-end of the E - Box motif	Vierbuchen et al., 2010; Karow et al., 2012; Wapinski et al., 2013; Yamamizu et al., 2013; Raposo et al., 2015; Soufi et al., 2015
FoxD3	- Hepatocyte initial specification, - Binding to the Alb1 enhancer in ESCs	Winged helix DBD	Clark et al., 1993; Ramakrishnan et al., 1993; Gualdi et al., 1996; Bossard and Zaret, 1998; Xu et al., 2007, 2009
FoxA1/A2	- Hepatocyte later specification and differentiation, replacement of FoxD3 at the Alb1 enhancer in definite endoderm; - Breast cancer (regulation of the estrogen response)	- Winged helix DBD; - Favors “accessible nucleosomes” - Genome scanning - H3K4me1/H3K4me2 binding	Xu et al., 2007; Magnani et al., 2011; Magnani and Lupien, 2014; Iwafuchi-Doi et al., 2016
Klf4	- iPSC reprogramming, - Different types of cancer - Cell lineage specification in the embryo	- Targeting of a degenerate hexameric motif (instead of canonical nonameric motif) at nucleosome-enriched sites	Takahashi and Yamanaka, 2006; Soufi et al., 2012, 2015
Oct4	- iPSC reprogramming, - Several types of cancer - Involved in the development of different cell lineages during embryogenesis	- Binding of separate half sites of the bipartite POU-domain (POUS/POUHD)	Takahashi and Yamanaka, 2006; Soufi et al., 2012, 2015
PU.1	- Hematopoietic lineages	Unknown	Smale, 2010; Barozzi et al., 2014
Sox2	- iPSC reprogramming - Neuronal fate specification	Recognition of a degenerate motif facilitating recognition of histone bound DNA minor groove	Takahashi and Yamanaka, 2006; Karow et al., 2012, 2014; Soufi et al., 2015
Pbx1	- Skeletal muscle lineage specification; - Breast cancer (regulation of the estrogen response); - Adult neurogenesis and neuronal lineage specification	- H3K4me1/H3K4me2 binding	Berkes et al., 2004; Maves et al., 2007; Magnani et al., 2011; Thiaville et al., 2012; Grebbin et al., 2016

Summarizes several established pioneer factors in comparison to PBX1. The list gives the physiological contexts in which priming was established and potential mechanisms that may mediate their priming function. For more detailed information on these proteins or for further discussion of general priming and pioneer factor mechanisms we would like to refer the reader to a series of excellent recent reviews (Iwafuchi-Doi and Zaret, 2014; Magnani and Lupien, 2014; Zaret and Mango, 2016).