Previous studies suggest the critical region leading to abnormalities in addition to α-thalassaemia is an 800 kb region between ~0.9 and~1.7 Mb from the telomere of chromosome 16 p18 and SOX8 has been proposed as the critical haploinsufficient gene.19 However, a report of a family with no developmental delay nor MR harbouring a 0.976 Mb deletion, suggests deletions of SOX8 may not lead to MR with complete penetrance and any ‘critical region’ for MR must start after this point20 (family ‘F’ in figure 2 and table 1). This evidence concerns the gene SOX8 and thalassemia.