Functional impact of SAMD9/SAMD9L mutants. (A) Doxycycline-induced expression of rTagRFP-SAMD9L protein in HEK293 cells. (B-C) Cell proliferation curves of HEK293 cells with inducible expression (wild-type [WT] or mutant) of SAMD9/SAMD9L proteins; SAMD9 (B), SAMD9L (C). SAMD9/SAMD9L WT protein (black circles) had slower cell growth than noninduced HEK293 cells (white circles). GOF SAMD9L variant His880Glu was used as a positive control of slower cell growth than WT. Growth curve data are representative of 3 independent assays performed in triplicate. Error bars indicate standard errors. (D) Model of SAMD9/SAMD9L GL variant’s physiology in MDS. GOF SAMD9/SAMD9L GL variants suppress proliferation of human hematopoietic stem/progenitor cells (HSPCs), could be removed by genetic reversions that are rapidly selected for in pediatric MDS without additional secondary hits (upper panel). In contrast, LOF SAMD9/SAMD9L GL variants found in adult MDS cases increase cell proliferation of HSPCs, were not subject to somatic reversion, and were accompanied by additional secondary hits. The greater number of hits needed explains the longer latency in adult MDS (lower panel).