Abstract Breast cancer is one of the most common causes of cancer mortality, with close to two million new cases and just over 600 000 cancer deaths projected in 2022 in the United States of America alone. Identifying prognostic markers for breast cancer is critical to reducing these numbers. Retinoblastoma binding protein 6 (RBBP6) is considered a potential cancer biomarker due to its association with cell proliferation and the fact that it is overexpressed at breast cancer sites where there is marked apoptosis and elevated p53. More information is emerging regarding the role of RBBP6 in cancer treatment, specifically its potential to sensitize cancer cells to radiation and certain chemotherapeutic agents. In vivo studies on RBBP6 in breast cancer mouse models are limited. Our study aims to investigate the molecular function and the prognostic potential of RBBP6 in breast cancer using MCF-7 cell line and the FVB/N-Tg(MMTV PyVT)634Mul/J breast cancer mouse model. The expressional effects of RBBP6 on cancer cell proliferation and metastatic tumour growth in cancer mice were examined in vitro and in vivo using RT-PCR, western blotting, and tumour size monitoring. The results demonstrate that RBBP6 mRNA expression was significantly higher in both local and distal tumours with a corresponding overexpression of angiogenic and metastatic key markers, VEGF164 and CD31, respectively. In addition, tumour growth reduced gradually over a period of two weeks in response to RBB6 gene knockdown. Our study indicated that RBBP6 may be a potential metastatic marker in breast tumours. Citation Format: Pontsho Moela, Mayur Govender, Melvin Ambele. Investigating the effects of in vivo silencing of RBBP6 gene on tumor growth in the FVBN-Tg(MMTV PyVT)634MulJ breast cancer mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2469.