AREG and neoplasm: This suggested that secretion of amphiregulin by the co-injected human fibroblasts plays a major role in establishing a tumor microenvironment that is enriched for activated fibroblasts, and since the co-injected human fibroblasts are almost entirely replaced by mouse fibroblasts at the point of excision (6 weeks), this effect must be propagated through recruited mouse fibroblasts.