GSTM1 and Schnyder corneal dystrophy: On the other hand, the high frequency of heterozygous profiles (Hb SC, Hb SD, Hb CC and Hb C/β-thal) detected in over 16-year-old subjects could be explained by the fact that these are almost asymptomatic forms of SCD, i.e. less expressive in terms of severity of manifestations.