## 1 Introduction  
Tumors in urinary-tract-related organs include renal carcinoma, which originates in the renal ureter; urothelial carcinoma, which originates in the urothel; and prostate cancer in men. Although all these organs are associated with the urinary tract, each tumor has a different histology, different developmental risks, and different treatment strategies. Urine cytological diagnosis can be a trigger for cancer detection. Renal, urothelial, and prostate cancer are often discovered incidentally. Recent improvements in the accuracy of diagnostic imaging have enabled the early detection of renal cancer and have increased the survival rate to more than 70%. However, because distant metastatic recurrence may occur after a long disease-free period, finding a radical cure is of great importance. Urothelial carcinoma may be detected in the presence of hematuria. Tumor cells in urine may be detected by cytology; however, in the case of highly differentiated urothelial carcinoma, it is difficult to distinguish these tumor cells from hyperplasia or papilloma. One major problem is that even if tumor cells are found in the urine they cannot be immediately diagnosed as cancerous. Prostate cancer is one of the most common cancers in the world for men, is the third most common cancer in terms of incidence, and usually progresses slowly and persists within the prostate [1]. However, some tumors are very aggressive and can rapidly metastasize to distant sites such as the bone, lungs, and liver. Serum prostate-specific antigen (PSA) is clinically useful as a marker for prostate cancer, but it does not express tumor characteristics; this means that it is difficult to determine whether follow-up or therapeutic intervention is required. Many approaches to detecting tumor-related genes based on genetic analyses have been applied. These include the detection of gene mutations based on somatic mutations in tumor cells, increased or reduced protein expression, the detection of fusion genes due to chromosomal translocations, or the detection of chimeric mRNAs or proteins expressed by fusion genes. Mutations in tumor-related genes often occur in signaling molecules that disrupt the growth signals of tumor cells. Therefore, they are used as companion diagnostics to evaluate the effectiveness of molecular targeted therapies, particularly inhibitors. In solid tumors, the most important molecules to target are EGFR, KRAS, BRAF, BRCA1/2, ALK, RET, MEK, and FGFR [2; 3; 4; 5; 6; 7; 8; 9; 10]. Although not clinically practical, a great number of reports present the potential use of microRNAs (miRNAs) as tumor markers based on their functional analysis, since their abnormal expression plays an important role in tumor cells [11; 12; 13; 14; 15; 16]. Their importance is also believed to be significant in urinary-tract-related tumors. miRNAs are low-molecular-weight RNAs consisting of 19-25 bases that bind to the 3\({}^{\prime}\) untranslated region of target mRNAs, inhibit transcription and translation, and degrade mRNAs, thereby contributing to in vivo processes such as development, cell proliferation, and differentiation [17; 18; 19; 20]. In various diseases such as cancer and metabolic, neurological, and infectious diseases, miRNAs greatly contribute to cell survival by suppressing or promoting gene expression [17; 19; 21]. In particular, miRNAs suppress or promote cancer cell proliferation. These miRNAs regulate many biological phenomena, and because they only recognize 7-8 nucleotides of their target mRNAs, the number of genes targeted by a single miRNA can be as many as several hundred. Various life phenomena and diseases, including cancer, in which miRNAs are involved, are extremely diverse (Figure 1). Therefore, when investigating their cellular functions--if the miRNAs are related to cancer from the viewpoint of abnormal expression of miRNAs--it is very useful to elucidate (a) the expression regulation of tumor suppressor genes or oncogenes using miRNAs and (b) the regulation of miRNA dynamics by tumor suppressor genes or oncogenes.