Determining the timing of CH clonal initiation. Two of the approaches used for determining the age at which CH clones emerge are construction of hematopoietic phylogenetic trees using passenger mutations as tracking barcodes (A) and studies of CH mutations in the blood DNA of monozygotic twins (B). These approaches have confirmed that mutations driving clonal growth, which become detectable in adulthood, can be acquired early in life and, in some cases, before birth.