In this context, a recent study by Sarvi and coworkers provided evidence that CD133+ cells isolated from SCLC cell lines exhibit properties of CSCs in that they form colonies in vitro and tumors in vivo, are resistant to anti-tumor agents such as etoposide and remain undifferentiated when grown in serum-free medium, while they differentiated in serum-rich medium [204]. The gene discussed is PROM1; the disease is neoplasm.