21nt

21nt is a term used in molecular biology and related fields to denote a sequence length of 21 nucleotides. The “nt” stands for nucleotides, the basic units of DNA and RNA. The notation is commonly applied when describing short oligonucleotides such as primers, probes, or therapeutic RNA molecules.

In practice, 21-nucleotide lengths appear in several contexts. Primer design for polymerase chain reaction (PCR) often targets oligos in the range of about 18 to 25 nucleotides, with 21 nt being a typical length that balances specificity and efficiency. Small interfering RNA (siRNA) and related gene-silencing technologies frequently use guides around 21 to 23 nt in length; the mature guide strand is typically about 21 nt, with the duplex companion also in a similar size range. In small RNA sequencing, reads corresponding to functional small RNAs are often categorized by length, with the 21-nt class being a common size.

Beyond biology, 21nt can appear as a product code, model number, or shorthand in contexts where a 21-nucleotide specification is relevant. Because “21nt” lacks a single universally fixed meaning, its interpretation depends on the surrounding context, and clarification is usually needed to determine the intended reference.

See also: Nucleotide, Oligonucleotide, siRNA, RNA interference, Primer design.