Mutations in the Euchromatic Histone Methyltransferase 1 (EHMT1) gene cause 
Kleefstra syndrome, a rare form of intellectual disability (ID) with strong 
autistic traits and sensory processing deficits. Proper development of 
inhibitory interneurons is crucial for sensory function. Here we report a 
timeline of Parvalbumin-positive (PV+) interneuron development in the three most 
important sensory cortical areas in the Ehmt1+/- mouse. We find a hitherto 
unreported delay of PV+ neuron maturation early in sensory development, with 
layer- and region-specific variability later in development. The delayed PV+ 
maturation is also reflected in a delayed maturation of GABAergic transmission 
in Ehmt1+/- auditory cortex, where we find a reduced GABA release probability 
specifically in putative PV+ synapses. Together with earlier reports of 
excitatory impairments in Ehmt1+/- neurons, we propose a shift in 
excitatory-inhibitory balance towards overexcitability in Ehmt1+/- sensory 
cortices as a consequence of early deficits in inhibitory maturation.