While initial works identified CD133 molecule as a reliable CSC marker in primary human colorectal cancers (O’Brien et al, 2007; Ricci-Vitiani et al, 2007), a subsequent study has shown that in both mouse and human colorectal cancers, CD133 expression is not restricted to rare cell subsets, but it is detectable in a large majority of tumour cells, irrespective of their tumourigenicity (Shmelkov et al, 2008). The gene discussed is PROM1; the disease is colorectal cancer.