The Human RBP4 Kit provides assay-specific components for the quantitative determination of endogenous retinol binding protein 4 (RPB4) in human serum, plasma, and cell supernatant. Retinol-binding protein 4 (RBP4) is a 21 kDa member of the lipocalin superfamily that transports Vitamin A (retinol) from liver stores to peripheral tissues via serum. The RBP4-retinol complex interacts with transthyretin (TTR) and prevents it from being filtered by the kidney. The C-terminal, processed forms of RBP4 do not bind TTR; they are excreted into the urine and, during renal failure, they accumulate in the serum. RBP4 also acts as an adipokine and has been linked to the development of obesity, type 2 diabetes (T2DM), and insulin resistance. Secreted by adipocytes and hepatocytes, RBP4 promotes hyperglycemia through downregulation of the glucose transporter type 4 (Glut4). Glucose transport via Glut4 is the rate-limiting step for glucose use by muscle and adipose tissue. These processes are impaired in adipocytes of obese individuals and those with T2DM. Elevated RBP4 in urine and serum often reflects the onset of cardiovascular complications and acute renal dysfunction associated with metabolic disorders.