KRAS and neoplasm: We previously reported that while CD44v6 is a functional marker that identifies tumour-initiating CR-CSCs, the CD44v6-negative population represents the progenitor and differentiated fraction.31 33 A cohort of 31 out of 47 primary CSphC lines showed that the high percentage of CD44v6 expression levels resided in the Ras/Braf-wt cells resistant to cetuximab, even though these expression levels are similarly distributed between Ras/Braf-wt, Braf-mutated and Kras-mutated cell lines (figure 1D and online supplemental figure 1E).