ARX and epilepsy: They are fast spiking interneurons with a high metabolic demand, which makes them vulnerable for injury.64 Interestingly, the development of SST and PV interneurons is depending on SHH levels,65,66 and deletion of Arx in PV neurons led to increased neuronal activity and altered synaptic properties in a mouse model.42 PV interneurons play a crucial role in spike timing of glutamatergic neurons, and abnormalities in PV signalling have been implicated in epilepsy, occasionally seen in 4H patients.