Using also invertor technology for performing a conditional expression of Mll-Af4 in lymphoid lineage in mice, Metzler et al. found the development of mature lymphoproliferative disease, demonstrating that the stem cell in which the MLL fusion protein is expressed is not an uncommitted progenitor and that MLL-AF4 influences the phenotype of the tumour when expressed within cells of the lymphoid lineage [17]. The gene discussed is AFF1; the disease is neoplasm.