A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes.

Following this discovery, Nirenberg, Philip Leder, and Gobind Khorana identified the rest of the genetic code and fully described each three-letter codon and its corresponding amino acid.

Gene syntax, defined by the order and orientation of genes within a plasmid, significantly influences gene expression levels, ratios, and variability, even when regulatory sequences remain unchanged.