Notwithstanding this variability, all complications result from polymerisation of the abnormal form of haemoglobin, HbS, present in patients' RBCs. HbS has a single amino acid substitution at a critical site on the haemoglobin molecule. At the β6 position, glutamic acid is replaced by valine and the loss of negative charge enables neighbouring HbS molecules to aggregate on deoxygenation, forming long rigid polymers which distort RBC shape and cause other deleterious abnormalities, including altered rheology, elevated membrane permeability and increased fragility