There are few instances in oncology where reciprocal clinical and laboratory translation studies have accelerated the understanding of disease biology and treatment more so than the decade following the Food and Drug Administration (FDA) approval of lenalidomide (RevlimidTM; Celgene Corporation, Summit, NJ, USA) for the treatment of patients with myelodysplastic syndrome (MDS) and chromosome 5q deletion (del(5q)). Lenalidomide was approved by the FDA in December 2005 on the merits of a multicenter phase 2 study, which demonstrated sustained and prolonged transfusion independence in the majority of participants. Since then, del(5q) MDS has emerged as one of the best characterized bone marrow malignancies and, in particular, has raised our understanding as to how allelic haplodeficiency underlies both its hematological phenotype and the selective sensitivity to lenalidomide by virtue of synthetic lethality. Herein, we review the clinical and biological discoveries that have advanced our understanding of del(5q) MDS and its treatment since its approval by United States and European regulatory agencies.