In fact, whereas either ‘hit’ alone was found to lead to an increase in tissue MC in visceral organs in mice without fulfilling histopathological criteria of SM, transplantation of NPM-ALK-transfected progenitors into lethally irradiated IL-9 transgenic mice was found to be associated not only with lymphoma-formation, but also with the development of a neoplasm resembling systemic mastocytosis, sometimes with aggressive destructive growth indicative of advanced SM. The gene discussed is IL9; the disease is systemic mastocytosis.