PROM1 is highly expressed by some primary tumors. (A): PROM1 expression was assessed by quantitative RT-PCR in 48 primary ESFT specimens. Levels of expression (normalized relative to GAPDH in the same sample) were < 1% in 37 tumors, 1-2% in 5 tumors, 2-3% in 2 tumors, and > 3% in 4 tumors (marked with arrows). Median expression of all 48 tumors was 0.06%. Similar expression data were generated when Ct values were normalized to expression of ACTIN instead of GAPDH (not shown). Two of 4 high-expressing cases demonstrated primary drug resistance resulting in early death. NED: no evidence of disease at last follow-up; DOD: dead of disease. (B): Translocation and tissue of origin data for the 48 tumors evaluated in (A). EWS-ERG+ fusions were significantly over-represented among the 11 PROM1 expressing tumors (p = 0.01, Fisher's exact test). (C): Frozen tumor sections obtained from a PROM1-negative and a PROM1-over-expressing tumor (highest expressing tumor from (A)) were stained for CD133. Only rare CD133+ cells were detected in the PROM1-negative tumor. In contrast, large numbers of CD133+ cells were detected in the chemoresistant PROM1-over-expressing tumor and the number of CD133+ cells increased post-therapy.