A recent study by Schneider et al has shown that CSNK1A1 plays a central role in the pathogenesis of del(5q) MDS.15CSNK1A1 encodes CK1α, a central regulator of β-catenin26 which is a major driver of stem cell self-renewal.15 Heterozygous inactivation of Csnk1a1 in mice led to β-catenin activation and expansion of HSCs,15 suggesting that CSNK1A1 haploinsufficiency may be the mechanism underlying the initial clonal expansion in patients with the 5q- syndrome. The gene discussed is CSNK1A1; the disease is myelodysplastic syndrome associated with isolated del(5q).