Human hair genealogy. a. Hair follicle genealogy can be divided into three phenotypic phases: neogenesis, bulge stem cell latency, and anagen differentiation and division. Mitotic age includes divisions during all three phases and may be inferred from numbers of replication errors (methylation) in sampled follicle cells. Short hairs should have fewer anagen divisions relative to longer hairs and therefore fewer errors (less methylation). Hairs from individuals of different ages have similar neogenesis and anagen intervals, and therefore any age-related increase in average methylation reflects lifelong stem cell mitotic activity. Average mitotic ages would be similar for hairs plucked from different aged individuals because of stem cell bulge latency. b. Sequences (after bisulfite treatment) of the CSX and SOX10 methylation tags. CpG sites that may be methylated are in bold, and converted Cs in non-CpG sites are capital T's. PCR primers are underlined. c. Examples of CSX tags sampled from two follicles (eight tags per hair) from a 37 year old. Circles represent the 5' to 3' order of adjacent CpG sites and filled circles are methylated.