SLC12A2 and early-onset autosomal dominant Alzheimer disease: Key findings demonstrate that Cl− dysregulation follows distinct patterns: (1) in epilepsy, KCC2 downregulation converts GABAergic inhibition to excitation, promoting seizures; (2) in Alzheimer’s disease (AD) models, pre-symptomatic KCC2 loss in hippocampus is observed, with KCC2 restoration reversing aspects of cognitive decline; (3) in autism spectrum disorders (ASD), developmental delays in GABA polarity shifts feature due to altered NKCC1/KCC2 ratios; and (4) in Huntington’s disease (HD), striatal neuron-specific Cl− imbalances are linked to motor dysfunction.