TKT and infection: For example, by scoring null mutations in the tk gene using ganciclovir, mutation rates in HSV-1 have similarly been estimated to range from 5.9 × 10−8 (Hwang et al. 2002; Drake and Hwang 2005) to 1.0 × 10−7 (Hall and Almy 1982) substitutions per nucleotide per cell infection, where cell infection is an estimate of a viral generation.