RBP4 and Bjornstad syndrome: Presence in the urine of retinol binding protein 4 (uRBP), a low molecular weight protein that is filtered at the glomerulus and almost completely reabsorbed via the proximal tubule,6, 7, 8 is a well‐recognised biomarker of PTD in diseases characterised by the renal Fanconi syndrome such as those associated with plasma cell dyscrasias.9