MTHFR and sickle cell disease: Furthermore, the heterozygous dominant (CT) variant of the MTHFR C677T gene polymorphism was about 34% and 2.2% for the homozygous recessive (TT) variant type in a survey of Brazilians with sickle cell disease [39] and this differs from a frequency of 15.6% for CT variant and 1% for the TT variant found in HbSS individuals in this study.