DNMT1 and cancer: Insertions in the exon hinder the coding sequence: (a) the insertion into introns may change splicing patterns, and insertion in the promoter or enhancer spoils transcription; (b) at the transcriptional level, piRNAs and piwi proteins directly modify chromatin structure and histone proteins in the nucleus via the regulation of DNA methyltransferase (DNMT); (c) piRNAs are involved in cancer regulation by altering the expression of cancer-related genes in a mechanism similar to miRNA and were shown to be regulated by transcriptional factors in human cancer [226,227,228].