SULT1B1 is a cytosolic sulfotransferase that catalyzes sulfate conjugation of diverse substrates using 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as the sulfate donor 1. The enzyme metabolizes small phenols (1-naphthol, p-nitrophenol), biogenic amines including dopamine, and thyroid hormones (T3, reverse T3, 3,3'-diiodothyronine) 1. SULT1B1 also sulfates 4-ethylphenol, a gut bacteria-derived tyrosine metabolite, producing 4-ethylphenol sulfate (4-EPS) that crosses the blood-brain barrier and may negatively regulate oligodendrocyte maturation. The enzyme is located on chromosome 4.1 2 and participates in xenobiotic metabolism and hormone regulation. Clinically, SULT1B1 variants affect enzyme kinetics; the L145V allelic variant, prevalent in African Americans (25% allelic frequency), exhibits reduced PAPS affinity and decreased catalytic efficiency 1. SULT1B1 expression is regulated by hypoxia-responsive pathways in hepatocellular carcinoma through histone H3Y99 sulfation, affecting HIF1A and PDK1 transcription 3. Genome-wide association studies link SULT1B1 to female infertility and endometriosis risk when influenced by endocrine-disrupting chemicals 45. Additionally, phthalate monoesters and chlorophenols competitively inhibit SULT1B1 activity, suggesting xenobiotic interactions 67. SULT1B1 exhibits tumor-suppressive functions in esophageal squamous cell carcinoma 8.