The increased activity of tumour ADA could be a reflection of ADA‐rich immune cell accumulation in malignant breast tumours.6, 15 As we reported previously that activated endothelial cells also exhibit strongly increased eADA activity, ADA in tumour could also derive from actively proliferating endothelial cells during tumour angiogenesis.16, 17 Because of the unclear data regarding the source of ADA in tumour development, this work focuses on cellular origin of its activity. Here, ADA is linked to breast cancer.