Background and Aims: A great number of studies have demonstrated that long non-coding RNAs (lncRNAs) play important roles in the occurrence and development of cancers in recent years. lncRNA RP1-85F18.6 is a newly discovered non-coding RNA, which is overexpressed in colon cancer tissues and cells, and can promote the proliferation and invasion, inhibit the apoptosis and regulate the cell cycle of the cancer cells. However, there are no reports of studies about the lncRNA RP1-85F18.6 in breast cancer so far. So, this study was conducted to preliminarily investigate the expression of lncRNA RP1-85F18.6 in breast cancer cells and its effect on proliferation and cell cycle.   
Methods: The expressions of lncRNA RP1-85F18.6 in breast cancer cell lines MDA-MB-231, MBA-MD-468, MCF-7 and normal mammary cell line MCF-10A were detected by qRT-PCR. MDA-MB-231 cells were transfected with RP1-85F18.6 silencing sequence (silencing group) and negative control sequence (negative control group), respectively, using the untreated MDA-MB-231 cells as blank control group. In the three groups of cells, the proliferation abilities were measured by MTT assay, the cell-cycle distributions were examined by PI staining flow cytometry, and protein expressions of Ki67, proliferating cell nuclear antigen (PCNA), cyclin A1 and p21c1p1 were determined by Western blot analysis. 
Results: The relative expression levels of lncRNA RP1-85F18.6 in MDA-MB-231, MBA-MD-468 and MCF-7 were all significantly higher than that in the normal mammary cell line MCF-10A (all P<0.05). The OD490 nm values at 24, 48, 72 and 96 h after transfection in silencing group were all significantly lower than those in blank control group (all P<0.05). In silencing group compared with blank control group, the proportion of G1/G0 phase cells showed no significant change (P>0.05), but the proportion of S phase cells was significantly increased, and the proportion of G2/M phase cells were significantly decreased (both P<0.05); the expression levels of Ki67, PCNA and cyclin A1 proteins were significantly down-regulated, while the expression of p21C1P1 protein was significantly up-regulated (all P<0.05). No statistical differences were noted in all above indexes between negative control group and blank control group (all P>0.05). 
Conclusion: LncRNA RP1-85F18.6 is highly expressed in breast cancer cells, which may participate the occurrence and development of breast cancer through regulating the expressions of proliferation-promoting and cell-cycle-controlling proteins. Intervention of lncRNA RP1-85F18.6 and its target genes may be a new therapeutic  approach for the treatment of breast cancer.