PTN protein is produced by myeloma cells. PTN protein expression was assessed in myeloma cell lines, the BMMCs from myeloma patients, and healthy donors by IHC. The myeloma cell lines RPMI 8226 and U266, and the human myeloma tumor LAGλ-1, expressed high levels of PTN protein. In contrast, the monocytic cell line THP-1 showed no PTN staining. BMMCs from a healthy donor (control) did not produce PTN. PTN was highly expressed in the BM from an MM patient with active disease and in a relapsing patient but not in an MM patient in remission.