We have reported that MM-1, a novel protein binding to the myc box II, suppressed transcription and transformation activities of c-Myc and that A157R mutation of c-Myc, which is observed at high frequency in patients with lymphoma or leukemia, abrogated all of the functions of MM-1 toward c-Myc, indicating that MM-1 is a novel tumor suppressor [11], [12]. This evidence concerns the gene PFDN5 and lymphoma.