BCDIN3D is an O-methyltransferase that functions as a cytoplasmic tRNA-His specific 5'-monomethylphosphate capping enzyme. The enzyme specifically monomethylates the 5'-monophosphate of cytoplasmic histidyl tRNA (tRNA-His), recognizing its unique structural features including the G-1:A73 mispair at the top of an eight-nucleotide-long acceptor helix 1 2. This methylation protects tRNA-His from degradation by shielding it from cleavage by DICER1 2. BCDIN3D also methylates the 5'-monophosphate of certain pre-miRNAs, though with lower efficiency, acting as a negative regulator of miRNA processing by reducing DICER1-mediated cleavage 2. The enzyme adopts a classical Rossmann-fold methyltransferase structure and uses S-adenosyl-L-methionine as a methyl group donor 1. BCDIN3D is clinically significant as it is overexpressed in breast cancer and associated with poor prognosis 1 2. The enzyme regulates glycolytic metabolism through the let-7 microRNA pathway, affecting Aldolase C expression 3. Genetic variants in BCDIN3D are associated with obesity-related phenotypes and weight gain 4, suggesting broader metabolic functions beyond RNA modification.