 Each nucleotide is subject to one transition (e.g., T to C) and 2 transversions (e.g., T to A or T to G).Because a site (or a sequence) is subject to twice as many transversions as transitions, the total rate of transversions for a sequence may be higher even when the rate of transitions is higher on a per-path basis.In the molecular evolution literature, the per-path rate bias is typically denoted by "κ" (kappa), so that, if the rate of each transversion is "u", the rate of each transition is "κu".