ARCH. ARCH is the clonal expansion of at least single HSPCs capable of multilineage differentiation. All cells belonging to the expanding clone share genetic variants, and at least some of them are recurrent and belong to ARCH-defining variants as in Table 1. A minimum of 1 somatic disrupting variant is needed for the diagnosis of ARCH, and wild-type cells that do not carry the mutation must be present. Although somatic mutations should be present both in hematopoietic stem and progenitor cells (HSPCs) and in mature cells, this concept is not mandatory for diagnosis because cell sorting is currently less feasible. Professional illustration by Somersault18:24.