The present application relates to a method for diagnosing a glucose transporter type 1 (GLUT1) deficiency syndrome that utilizes polypeptides derived from the soluble part of the glycoprotein of the enveloped virus of primate T-cell leukemia virus (PTLV). The polypeptides, named receptor binding domain ligands (RBD), are selected for their ability to bind specifically to GLUT1. The method involves determining the level of GLUT 1 expression at the cell surface and comparing the level to a reference value.
