Circular RNAs (circRNAs) are pervasively expressed circles of non‑coding RNAs. Even though many circRNAs have been reported in humans, their expression patterns and functions remain poorly understood. In this study, we employed a pipeline named RAISE to detect circRNAs in RNA‑seq data. RAISE can fully characterize circRNA structure and abundance. We evaluated inter-individual variations in circRNA expression in humans by applying this pipeline to numerous non‑poly(A)-selected RNA‑seq data. We identified 59,128 circRNA candidates in 61 human liver samples, with almost no overlap in the circRNA of the recruited samples. Approximately 89% of the circRNAs were detected in one or two samples. In comparison, 10% of the linear mRNAs and non‑coding RNAs were detected in each sample. We estimated the variation in other tissues, especially the circRNA high-abundance tissues, in advance. Only 0.5% of the 50,631 brain circRNA candidates were shared among the 30 recruited brain samples, which is similar to the proportion in liver. Moreover, we found inter- and intra-individual diversity in circRNAs expression in the granulocyte RNA‑seq data from seven individuals sampled 3 times at one-month intervals. Our findings suggest that careful consideration of inter-individual diversity is required when extensively identifying human circRNAs or proposing their use as potential biomarkers and therapeutic targets in disease.

Invasion and metastasis are the basic characteristics and important markers of malignant tumors, which are also the main cause of death in cancer patients. Epithelial–mesenchymal transition (EMT) is recognized as the first step of tumor invasion and metastasis. Many studies have demonstrated that cell fusion is a common phenomenon and plays a critical role in cancer development and progression. At present, cancer stem cell fusion has been considered as a new mechanism of cancer metastasis. Mesenchymal stromal/stem cell (MSC) is a kind of adult stem cells with high self‐renewal ability and multidifferentiation potential, which is used as a very promising fusogenic candidate in the tumor microenvironment and has a crucial role in cancer progression. Many research results have shown that MSCs are involved in the regulation of tumor growth and metastasis through cell fusion. However, the role of cell fusion between MSCs and malignant cells in tumor growth and metastasis is still controversial. Several studies have demonstrated that MSCs can enhance malignant characteristics, promoting tumor growth and metastasis by fusing with malignant cells, while other conflicting reports believe that MSCs can reduce tumorigenicity upon fusion with malignant cells. In this review, we summarize the recent research on cell fusion events between MSCs and malignant cells in tumor growth and metastasis. The elucidation of the molecular mechanisms between MSC fusion and tumor metastasis may provide an effective strategy for tumor biotherapy.