As a function unknown gene, C1orf109 is lower expression in various cells. Here, we reported that C1orf109L, the longest variant of C1orf109, which interacted with R-loop-regulating proteins to trigger R-loop, a three-stranded nucleic acid structure frequently mediated genome instability, accumulation. C1orf109L induce chronic DNA damage to promote P21 upregulation and strongly inhibits cell growth in vitro and in vivo by arresting the cell cycle in the G2 phase. With camptothecin (CPT), an R-loop activator, treatment, C1orf109L further triggers R-loop accumulation-induced DNA damage and promotes cell death by activating cell-death pathway. Furthermore, CPT treatment increases C1orf109L ubiquitination and turnover, which inhibits cell death and promotes the G0/G1 phase of the cell cycle. Therefore, our data illustrated the mechanisms underlying C1orf109L-related cell growth inhibition and provide feasibility and limitations for C1orf109L as a potential target for cancer therapy. Cell proliferation depends on the cell cycle. In abnormal cells, such as benign tumor cells and cancer cells, cell-cycle dis-regulation promotes cell growth [1]. For the cancer therapy strategies are mainly to kill cancer cell or inhibit cancer cell growth. Inducing cancer cell DNA damage is an efficient way to induce cancer cell death or growth inhibition. When genomic DNA is damaged, many pathways involved in cell-cycle regulation are activated, the most important of which involves activation of the DNA-damage checkpoint to arrest the cell cycle at different stages, including G1, S, and G2/M, which ensures that the damaged cell has enough time to repair DNA damage [2,3]. If DNA damage is so severe or the rate of damage is faster than the rate of the repairing process, then the cellular-death pathway is activated [4]. Among the DNA-damage checkpoints, the G1 and G2/M checkpoint restricts mitosis onset in response to multiple exogenous and endogenous factors [5]. With respect to endogenous factors, several proteins act as RNA-processing regulators that control the cell's fate, while dysfunction of RNA-processing-related proteins could trigger R-loop accumulation, and RNA-DNA hybrids can lead to DNA damage and induce-cell cycle arrest, which is an important factor for genome instability in abnormal cells, especially in cancer cells [6,7]. The R-loop also controls gene expression [8] and immunoglobulin (Ig)G class switching [9]. However, the regulation of R-loop proteins is poorly explored. Thus, in this study, we identified the novel gene, c1orf109, which is downregulated in a variety of cell lines. C1orf109L-eGFP expression triggers R-loop accumulation to induce DNA damage and arrest the cell cycle. We report that C1orf109L interacted with R-loop associated proteins and located on R-loops. C1orf109L overexpression that triggered R-loop accumulation no matter with or without CPT treatment in cells. The accumulated R-loops induced DNA damage which inhibits cell growth in vitro and in vivo. In terms of the mechanism underlying cell growth, t...