Retinol binding protein 4, also known as RBP4, is a transporter protein[5] for retinol (vitamin A alcohol). It is mainly, though not exclusively, synthesized in the liver and circulates in the bloodstream as a hepatokine bound to retinol in a complex with transthyretin. RBP4 has been a drug target for ophthalmology research due to its role in vision. RBP4 may also be involved in metabolic diseases as suggested by recent studies. Retinol-binding protein 4 in urine (uRBP4) is a functional biomarker of disease of the proximal renal tubule1. When proximal tubular dysfunction interferes with reabsorption of proteins filtered by the renal glomerulus, striking increases of uRBP4 are found.