e23202 Background: The impaired stability of chromosomal structure promotes the abnormal segregation of chromosomes, increasing the risk of carcinogenesis. Chromosomal stability during segregation is managed by appropriate methylation in the centromeric region of chromosomes. Satellite alpha transcripts (SatA) is a non-coding RNA transcribed from the centromere, which could be involved in this machinery. We reported that overexpression of SatA led to chromosomal instability, which was observed in not only cancer regions (CR) but also noncancerous regions (NCR). It was enhanced in NCR of patients with multiple primary malignancies (MPM), suggesting the involvement of SatA in genomic damage in regions where multiple cancers may arise. In this study, we determined whether SatA is involved in breast cancer development with bilateral or MPM. Methods: We constructed lentiviral vectors expressing SatA and control viruses, which were used to infect human mammary epithelial cells. SatA levels were determined by real-time quantitative PCR (qRT-PCR) and copy number alterations were evaluated by array comparative genomic hybridization (CGH). Then, we investigated the levels of SatA in clinical specimens in 135 breast cancer (BC) patients including 128 tumors and 83 matched normal tissues. Results: qRT-PCR showed higher expression levels of SatA in cells infected with SatA virus than in the control. Array CGH analysis indicated frequent copy number alterations at several specific chromosomes in infected cells, despite no alterations in the control. Among the 135 patients, 24 had bilateral breast cancer (BBC) and 18 had MPM. SatA was much higher in CR in BBC and in MPM than in SBC. SatA was also higher in NCR in MPM than without MPM. Considering human epidermal growth factor receptor type2 (HER2), hormone receptor (HR), family history and other factors, the levels of SatA in CR were a significant independent factor predicting the development of BBC patients. Conclusions: Chromosomal instability induced by SatA is involved in the development of BBC and BC with MPM.