MIR21 and cancer: When upregulated in cancer cells, miR-21 inhibits numerous tumor suppressor mRNAs, which is associated with proliferation, apoptosis, and invasion.26, 27, 28 As miR-21 is the most abundant miRNA across cancer transcriptomes, and MIR21 knockout resulted in a reduced tumor growth, ciRSs were designed to impair miR-21 activities in vivo.9