Using this system, it was shown that human tumor cells growing in ESC-derived microenvironment are heterogeneous, with a CD44+ALDH1+ progeny capable of sustaining tumor growth in vivo: these cells generate a more differentiated cell progeny CD44− that, in turn, is able again to de-differentiate originating a self-renewing CD44+ cell population [216]. The gene discussed is CD44; the disease is neoplasm.