Efficient knockdown of miR-21 by CHAIN restored programmed cell death protein 4 (PDCD4) and reversion‐inducing‐cysteine‐rich protein with Kazal motifs (RECK) and further crippled downstream matrix metalloproteinases-2 (MMP-2), which undermined cancer proliferation, migration and invasion. Here, PDCD4 is linked to cancer.