Young age can be an independent prognostic factor for adverse prognosis in women with breast carcinoma (BC). In younger women, BC exhibited more aggressive pathological features than older women, indicating differences in biology. Frequent alterations in chromosomal (chr.) 3p22.3 in different malignancies indicated the existence of multiple candidate tumor suppressor genes (TSG) in this region, yet its association with BC remains unclear. In an effort to understand the differences in molecular pathogenesis in two age groups of BC, detailed analysis of alterations at chr.3p22.3 region was carried out in 47 early onset (group-A: 40 years) and 59 lateonset (group-A: >40 years) BC samples. Deletion mapping of the four candidate TSG, hMLH1, APRG1, ITGA9 and RBSP3/HYA22, located within 1 Mb of chr.3p22.3 showed high deletion in hMLH1 and RBSP3/HYA22 genes. Frequent methylation was also observed in these genes and significantly associated with their deletion. Quantitative messenger RNA (mRNA) expression and immunohistochemical analysis showed down-regulation of these genes. Alterations (deletion/methylation) of hMLH1 were significantly associated with RBSP3/HYA22 in group-A (P = 0.02). Significant poor survival in group-A patients with alterations in hMLH1 and RBSP3/ HYA22 and the same in group-B patients with hMLH1 alterations indicated their importance as prognostic markers. Differential association of alterations of these genes with higher histological grades, more advanced stages and positive lymph node involvement were also seen. Thus, the present study suggests hMLH1 and RBSP3/HYA22 to be candidate TSG associated with development of both early and late-onset BC undergoing frequent genetic and epigenetic alteration and having significant prognostic implications. (Cancer Sci 2008; 99: 1984-1991 B reast cancer (BC), is the third most prevalent malignancy and primary cause of death in women worldwide.(1) In India, it accounts for 19% of all cancers among women, and is the second most common cancer when both sexes are considered together, whereas about 23% of eastern Indian women suffer from this disease. (2,3) Considering the age at onset of tumor, BC can be classified as early onset (group-A: ≤40 years) and late-onset (group-B: >40 years).(4,5) However, the cut-off value for early onset BC varies among investigators, ranging from 35-50 years. (6,7) Despite this discrepancy, younger women with BC exhibit more aggressive pathological features, including large tumor size of higher grade, presence of positive lymph nodes, absence of steroid receptors and high S-phase fraction, than older women with BC.(6,7) Another report showed that, in an Asian population, BC patients below 40 years have tumors with a poorer prognostic profile. However, this did not translate into poorer overall survival and might be attributable to more aggressive adjuvant treatment of younger patients.(8) All these reports suggested early onset BC to be a biologically separate disease that independently predicts more adverse outcomes. Thus, the mol...