TUBB4B and retinitis: In the α/β-tubulin heterodimer, α-tubulin has a non-hydrolysing and non-exchangeable “N-site” which binds to GTP, whereas β-tubulin has an exchangeable “E-site” where GTP can be hydrolysed into GDP.46,47 Recently, McFadden and colleagues reported the p.(Glu11Arg) variant in TUBB4B, observed in a syndromic presentation of hearing loss, hyperopia without retinal abnormalities, renal tubular Fanconi Syndrome and hypophosphatemic rickets.