Table 1.

Different experimental techniques associated with controversy regarding existence of MSCs^*

	Studies Identifying MSCs						Authors Questioning The MSC Model
Study	Schatton et al. Nature 2008	Kupas et al. J Invest Dermatol. 2011	Boiko et al. Nature 2010	Civenni et al. Cancer Res. 2011	Boonyaratanakornkit et al. J Invest Dermatol. 2010	Luo et al. Stem Cells 2012	Quintana et al. Nature 2008	Quintana et al. Cancer Cell 2010
Marker	ABCB5	RANK (ABCB5+)	CD271	CD271	ALDH	ALDH	Multiple	CD271, ABCB5
Trypsin Dissociation	No	No	No	No	No	No	Yes	Yes
Sorting Technique	Magnetic Beads	FACS	FACS	FACS	FACS	FACS	FACS	FACS
Matrix	None	None reported	Matrigel	Matrigel	None reported	Matrigel	Matrigel	Matrigel
Host Strain	NOD/SCID	NSG	Rag2−/−gc−/−	NOD/SCID NSG	NOD/SCID NSG	NOD/SCID NSG	NSG	NSG
Transplant Rounds	2	1	2	3	2–3	3	1–2	2
Marker(+)MSC Enrichment	Yes	Yes	Yes	Yes	Yes	Yes	No	No
Genetic Lineage Tracing	Yes	No	No	No	No	No	No	No

^*Noteworthy differences in protocols include the use of trypsin, the degree of immunosuppression in animal xenograft carriers, and use of the melanoma cell stimulant, laminin, in the form of Matrigel. In the absence of trypsin to harvest cells (trypsin cleaves ABCB5 and CD271 from cells, as shown by Civenni et al. (Civenni et al., 2011)), multiple independent researchers have observed the existence of cells that comply with the definition of MSCs (there is a potential for false negatives due to trypsin proteolytic cleavage of cell-surface markers). Comparison of highly immunocompromised NSG mice to NK-depleted nude or NOD/SCID mice has also revealed that in the former (1) the parental melanoma heterogeneity could not be recapitulated, and (2) CD271− cells may form tumors in the first round but may not be serially passaged, as would be anticipated for MSCs.