miR-21 has been shown to target tumour suppressor genes, such as PDCD4 (programmed cell death 4), ANP32A (acidic nuclear phosphoprotein 32 family member A), SMARCA4 (SWI/SNF related, matrix-associated, actin-dependent regulator of chromatin, subfamily A, member 4), PTEN (phosphatase and tensin homolog), and SPRY2 (Sprouty RTK signaling antagonist 2), and its inhibition leads to significantly reduced tumour cell proliferation and decreased tumour growth in immunodeficient mice [62,65,70,71,72,73,74,75,76,77,78]. This evidence concerns the gene PTEN and neoplasm.