D6D (Δ6-desaturase, also known as FADS2) is a fatty acid desaturase responsible for catalyzing the first desaturation step of polyunsaturated fatty acids such as linoleic acid and α-linolenic acid. It consists of multiple functional domains and is primarily located in the endoplasmic reticulum membrane. D6D plays a crucial role in regulating the fluidity and signaling of cell membranes and promoting the synthesis of long-chain polyunsaturated fatty acids. The expression of D6D is tissue-specific, and mutations or dysregulation of its expression are associated with various diseases, including cardiovascular diseases, metabolic syndrome, and neurodegenerative diseases. The activity of D6D is regulated by multiple factors, affecting cellular lipid metabolism and physiological function.