Retinol binding protein 4, plasma, also known as RBP4, is a protein[1] that in humans is encoded by the RBP4 gene. Retinol-binding protein 4 has recently been described as an adipokine that contributes to insulin resistance in the AG4KO mouse model.[4] It is secreted by adipocytes, and can act as a signal to other cells, when there is a decrease in plasma glucose concentration. Mutations in the RBP4 gene have recently been linked to a form of autosomal dominant microphthalmia, anophthalmia, and coloboma (MAC) disease.