A cassette, consisting of the CAG (CMV-chicken beta actin) promoter, a splice acceptor site, a floxed pgk-neo stop construct, and the mouse Mir21 sponge sequence, was inserted via homologous recombination. The sponge sequence contained seven Mir21 binding sites (sequence UCAACAUCAGGACAUAAGCUA). This sequence acts as a competitive inhibitor for the microRNA.