The expression of microRNA in wild-type mouse heart. 2A. Approximately 50 microRNAs are cardiac-enriched microRNAs that are highly expressed in the adult mouse heart. 2B. The 50 cardiac-enriched microRNAs account for approximately 80% of the total microRNA expression in the mouse heart, whereas the remaining 528 miRNAs account for only 20% of the total microRNA expression. 2C. The expression levels of 50 cardiac-enriched microRNAs and U6-snRNA (reference gene). miR-1 ranks number 1 in expression, miR-133a ranks number 7 in expression. The expression level of miR-499 is similar to that of U6-snRNA. 2D. Differential expression of pri-miRNA transcripts. Both pri-mir-1-1 and pri-mir-1-2 are processed into mature miR-1, but pri-mir-1-1 transcript level is 6-fold higher than that of pri-mir-1-2 (n = 3, p < 0.05*). The pri-mir-133a1 level is lower than that of pri-mir-133a2 (n = 3, p < 0.05*). These data suggest that the contribution of each pri-miRNA to the mature miRNA pool may be different.