GSTM1 and autosomal dominant cerebellar ataxia: In SCA, however, KCC activity is abnormally high, and its aberrant response to deoxygenation – an increase in activity as oxygen tension falls from the PO2 of Hb to 0 mmHg – may be explained if polymerisation of HbS removes the source of deoxyHb for WNK displacement leading to decreased WNK activity.