Nosology of CH. CH is the expansion of a clonal population of HSPCs. Many instances of CH are caused by unknown drivers and can be referred to as CHUD (dotted branch). When molecular drivers of CH are known, they can be further subclassified as initiating events of myeloid malignances (myeloid CH) or lymphoid malignancies (lymphoid CH). Myeloid CH is classified as CH caused by somatic mutations in myeloid malignancy-driver genes at VAF ≥ 2%. The term CHIP is used in the absence of cytopenias, and CCUS is used when cytopenias are present. Mosaic chromosomal alterations (mCAs) can also be drivers of myeloid CH (m-MCA). Lymphoid CH is subdivided into CH caused by mutations in drivers of lymphoid malignancy with a VAF ≥ 2% (L-CHIP) or mCA that reflect chromosomal abnormalities driving lymphoid malignancies (L-mCA). Ever-improving sensitivity of next-generation sequencing has led to an increasing identification of low-abundance (VAF < 2%) clones, which some have referred to as micro-CH. The clinical significance of these low-abundance clones remains to be fully elucidated.