VDR and Osteopenia: Furthermore, VDR mice deficient in the gene encoding the vitamin D receptor (Vdr) (VDR knockout [KO] mice) also exhibit aging phenotypes (Haussler et al., 2010; Keisala et al., 2009) and 1,25(OH)2D3 can increase levels of the protein Klotho that has enzyme/coreceptor properties and that protects against aging phenotypes such as skin atrophy, osteopenia, and atherosclerosis (Haussler et al., 2016).