DNMT3Agene mutations, detected in 20-25% ofde novoacute myeloid leukemia (AML) patients, are typically heterozygous. Biallelic variants are uncommon, affecting ~3% of cases and identifying a worse prognosis. Indeed, two concomitantDNMT3Amutations were recently associated with shorter event-free survival and overall survival in AML. We present an AML case bearing an unusualDNMT3Amolecular status, strongly affecting its function and strangely impacting the global genomic methylation profile. A 56-year-old Caucasian male with a diagnosis of AML not otherwise specified (NOS) presented a complexDNMT3Amolecular profile consisting of four different somatic variants mapping on different alleles(in trans). 3D modelling analysis predicted the effect of theDNMT3Amutational status, showing that all the investigated mutations decreased or abolished DNMT3A activity. Although unexpected, DNMT3A’s severe loss of function resulted in a global genomic hypermethylation in genes generally involved in cell differentiation. The mechanisms through which DNMT3A contributes to AML remain elusive. We present a unique AML case bearing multiple biallelicDNMT3Avariants abolishing its activity and resulting in an unexpected global hypermethylation. The unusual DNMT3A behavior described requires a reflection on its role in AML development and persistence, highlighting the heterogeneity of its deregulation.