The active vitamin D metabolite 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3), Calcitriol) is a major regulator of gene expression in higher organisms. Protein abundance is an endpoint of gene expression that results from the balance between induction and degradation and is essential for adequate cell function. Proteins are degraded by proteases whose activity is in turn controlled by a number of endogenous protease inhibitors. 1,25(OH)(2)D(3) regulates several proteases and protease inhibitors in different cell types, putatively contributing to its regulatory effects of cell physiology. We have recently shown that 1,25(OH)(2)D(3) strongly induces the expression of cystatin D, an inhibitor of several cysteine proteases of the cathepsin family. Cystatin D induction may contribute to the antitumor effect of 1,25(OH)(2)D(3) against colon cancer by mechanisms that are both dependent and independent of cathepsin inhibition. Transcriptomic studies suggest that 1,25(OH)(2)D(3) also modulates the function of the ubiquitin-proteasome system. Thus, proteases and protease inhibitors are candidates to mediate to a certain extent the complex action of 1,25(OH)(2)D(3) in cancer cells.