Osteoclast gene expression in differentiated proliferative monocyte (PM) and non-proliferative (NP) subpopulations. NP and PM subpopulations, sorted as in Figure 1, were cultured for 14 days in macrophage colony-stimulating factor (8,000 U/mL) and receptor activator of nuclear factor-kappa-B ligand (50 ng/mL). Calcitonin receptor (CTR) and cathepsin K (Cath K) mRNA expression were measured by quantitative polymerase chain reaction. Samples from four individual donors were tested in triplicate, and data were normalized to 18S expression for each gene. Values are means of cycle threshold (Ct) numbers that were obtained in each sample ± standard error. The mean values for the PM population were significantly lower than those for the correspondingly treated NP population from the same donor (P ≤ 0.05).