Sterile α motif domain–containing 9 (SAMD9) and SAMD9-like (SAMD9L) syndromes are inherited bone marrow failure syndromes known for their frequent development of myelodysplastic syndrome with monosomy 7. In this issue of the JCI, Abdelhamed, Thomas, et al. report a mouse model with a hematopoietic cell–specific heterozygous Samd9l mutation knockin. This mouse model resembles human disease in many ways, including bone marrow failure and the nonrandom loss of the mutant allele. Samd9l-mutant hematopoietic stem progenitor cells showed reduced fitness at baseline, which was further exacerbated by inflammation. TGF-β hyperactivation was found to underlie reduced fitness, which was partially rescued by a TGF-β inhibitor. These findings illustrate the potential role of TGF-β inhibitors in the treatment of SAMD9/SAMD9L syndromes.