The increased concentrations of fibrosis markers (hydroxyproline and collagen type 1) observed during HP development, and above all, factors promoting pathological tissue repair (FGF2 and TGFβ), the levels of which were inversely dependent on the pulmonary calcitriol concentration, prompted us to take a closer look at the vitamin D3 impact on the phenomenon of epithelial-mesenchymal transition (EMT), which underlies tissue fibrosis [50,51]. The gene discussed is TGFB1; the disease is hypersensitivity pneumonitis.