HBB and Schnyder corneal dystrophy: Although there are several key phenotypes (anaemia, stroke, infections), foetal haemoglobin (HbF) has emerged as a central disease modifier; importantly, the expression of this modifier is amenable to therapeutic manipulation.11,12 Genetic variants at three principal loci, BCL11A, HBS1L-MYB and the HBB cluster account for 10–20% of HbF variation among SCD patients in the USA, Brazil and the UK.8,9