MicroRNAs play significant roles in development, metabolism and carcinogenesis, however, limited information is available about their primary transcripts and the transcriptional regulation of the microRNA genes. We report here the cloning of two primary miRNAs (pri-miR-433 and pri-miR-127) encoded by the miR-433-127 locus. Using both database mining and experimental methods, we isolated the full-length primary transcripts of the mouse miR-433 and miR-127 and demonstrated that they overlapped in a 5′-3′ unidirectional way. These two miRNA genes are expressed in a compact space by using overlapping coding regions. This is the first report to identify an economical structure for miR-433 and miR-127 genes, which may be a novel way of miRNA gene to maximize the genetic information in order to fit the complex physiological function of mammalian organism.