Development of cancer is a complex process involving multiple genetic and epigenetic alterations. In our microarray analysis of 81 breast carcinoma specimens, we identified peptidase domain containing associated with muscle regeneration 1 (PAMR1) as being frequently suppressed in breast cancer tissues. PAMR1 expression was also reduced in all tested breast cancer cell lines, while PAMR1 was expressed moderately in normal breast tissues and primary mammary epithelial cells. DNA sequencing of the PAMR1 promoter after sodium bisulfite treatment revealed that CpG sites were hypermethylated in the breast cancer tissues and cell lines. PAMR1 expression was restored by 5-aza-2' deoxycytidine treatment, demonstrating that promoter hypermethylation contributed to PAMR1 inactivation in the breast cancer cells. In addition, ectopic expression of PAMR1 markedly suppressed cancer cell growth. In summary, our study identified PAMR1 as a putative tumor suppressor which was frequently inactivated by promoter hypermethylation in breast cancer tissues.