To directly examine the role of ILC2s and IL-33 in cancer development, we conducted a comparison of the progression of the primary (TC1) or metastatic (A9) tumours, with and without IL-33-gene complementation, in mice transplanted with bone marrow (BM) from either wild type (WT) mice or mice lacking ILC2s (RORα−/−). The gene discussed is IL33; the disease is cancer.