Very rare, most cases previously reported do not meet WHO criteria.²⁶  Cytogenetics: no specific abnormality. Approximately 25%-30% of cases reportedly have clonal chromosomal abnormalities including del(20q), del(11q), or trisomy 8, 9, or 21.^1,45,47  Molecular genetics: JAK2 V617F reported; incidence unknown.⁴⁵