Schematic depicting the p21 (cyclin-dependent kinase inhibitor 1A gene, also known as CDKN1A ) locus. The blue lines, boxes and arrows at the top of the image show the p21 and antisense transcripts. Small black boxes connected by black lines depict the five primer pairs used in this study. THP-1 small RNAs (sRNAs) are shown as a density plot in red, followed below by RNA polymerase II (RNAPII) binding from multiple cell types, and CCCTC-binding factor (CTCF) binding across eight cell types. The THP-1 nuclear sRNA deep sequencing data is the deepest sRNA dataset currently available and was therefore, given the conservation of the CTCF site, was used as the basis for the sponge and mimic constructs in all cell lines.