The nm23 protein is the collective designation for two closely related proteins encoded by the genes nm23H1 and nm23H2 now called NME1 and NME2. The protein is expressed in the nucleus and cytoplasm of all normal cells, and on the cell surface of many haematopoietic cells, including erythrocytes. Tumour cells with a high metastatic potential often lack or express only a low amount of nm23 protein, hence the nm23 protein has been described as a metastasis suppressor protein. Recent studies, however, suggest a more complex relationship between occurrence of the nm23 protein and metastatic activity.