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Cutaneous mast cell tumours (MCTs) are among the most important skin tumours in dogs. Apart from c-KIT mutations, which are present in <18% of MCTs, little is known of the mechanisms of MCT development and independent growth of tumour cells. Recently, the α-subunit (CD25) of the interleukin (IL)-2 receptor (IL-2R) has been found to be expressed by canine cutaneous MCTs and this expression is negatively correlated with tumour grade. We thus hypothesized that the other two subunits of the IL-2R and the ligand IL-2 are also expressed and that IL-2-dependent pathways may have an impact on MCT development and independent tumour cell growth. Messenger RNA and protein expression levels of the IL-2R β-subunit (CD122), the IL-2R γ-subunit (CD132) and IL-2 were analyzed in canine cutaneous MCTs and compared with tumour grade and c-KIT mutation status. Eighty-six percent of the tumours expressed both subunits of the IL-2R and 64% expressed IL-2. In addition, neoplastic mast cells seem able to bind IL-2. IL-2Rγ and IL-2 protein expression levels were significantly decreased in higher grade tumours and IL-2 expression was significantly decreased in c-KIT mutated tumours. Thus, expression of the complete IL-2R and its ligand by canine cutaneous MCTs indicates a potential impact of IL-2R signalling in MCT development and tumour cell proliferation. The decrease in IL-2R expression with increasing histological evidence of malignancy suggests that the IL-2R may be more relevant for early MCT development and well-differentiated tumours.