In comparison to the young women, the elderly women showed decreased <i>VDR</i> and increased <i>CaSR</i> mRNA expression in PBMCs, as well as higher monocyte apoptosis.<h4>Conclusions</h4>Calcitriol administration increased both <i>VDR</i> and <i>CaSR</i> mRNA expression in PBMCs and decreased PBMC apoptosis.<h4>Conclusions</h4>The results obtained support the role of the vitamin D endocrine system as a regulator of the immune response and thus may contribute to explaining certain aspects of the immune dysfunction reported in individuals with vitamin D insufficiency. The gene discussed is CASR; the disease is immune system disorder.