2r

2R is a designation used in different fields to denote either a stereochemical configuration at a chiral center or a historical hypothesis about genome evolution in vertebrates. In organic chemistry, 2R refers to the absolute configuration at the second stereogenic center of a molecule, assigned according to the Cahn–Ingold–Prelog priority rules. It is typically part of a full descriptor such as (2R,3S)-2,3-dihydroxybutane. The 'R' labels indicate rectus (Latin for right) orientation of substituents around that carbon. When only a single stereocenter is specified, 2R effectively communicates the configuration at that center.

In genetics, 2R stands for two rounds of whole-genome duplication in early vertebrate evolution, a historical hypothesis known as the 2R hypothesis. According to this view, the ancestral vertebrate lineage underwent two successive genome duplications, expanding gene families (for example, Hox genes) and contributing to complexity. The 2R scenario has influenced interpretations of vertebrate genome structure, but its timing and universality remain topics of ongoing research and debate, with some lineages showing alternative or additional duplication histories.

Apart from these uses, 2R may appear in various context-specific codes, identifiers, or product names, where its meaning is defined by the relevant domain. In any case, full interpretation requires accompanying context or specification.