In 2009, we demonstrated that a peptide, which we named “Peptide A”, derived from the extracellular domain of T-cell leukemia translocation-associated gene (TCTA) protein, inhibited both RANKL-induced human osteoclastogenesis and pit formation of mature human osteoclasts. Here, we examined the effect of Peptide A on the cell proliferation of cell lines of small-cell lung carcinoma, breast cancer, and prostate cancer: RERF-LC-MA, MCF-7, and PC-3, respectively. Peptide A inhibited the proliferation of RERF-LC-MA, but not MCF-7 or PC-3. TCTA protein was immunohistologically detected in RERF-LC-MA and MCF-7. Thus, Peptide A may provide a novel strategy for the therapy of the patients with small-cell lung carcinoma, especially with bone metastasis. In addition, Peptide A may be useful for the treatment of various cancer patients with bone metastasis.