An osteoclastic protein-tyrosine phosphatase, PTP-oc, shares the same gene with a renal PTP, Glepp1. This study demonstrated that targeted deletion of PTP-oc promoter by homologous recombination in RAW264.7 cells completely abolished PTP-oc expression without affecting Glepp1 expression. This strategy to inhibit PTP-oc function has three advantages over commonly used gene knock down strategies (e.g., small interference RNA). This strategy: (1) yielded cells completely devoid of PTP-oc, (2) had no off-target gene silencing effects, and (3) did not affect Glepp1 expression. The inability of PTP-oc-deficient RAW264.7 cells to undergo RANKL-mediated osteoclastic differentiation confirmed a regulatory role for PTP-oc in RANKL-mediated osteoclast differentiation.