MC903 is a low-calcemic vitamin D3 analogue that is an approach used to treat patients with psoriasis, but can induce dermatitis as a side-effect, and application to murine skin induces dermatitis-like inflammation characterized by spongiosis, skin thickening, hypervascularisation, epidermal hyperplasia, pruritus, immune cell infiltration and dependence on type-2 cytokine pathways including type-2 inducer cytokines TSLP and IL-3345,46. The gene discussed is TSLP; the disease is psoriasis.