During development, some neurons can change their sensitivity to neurotransmitters such as GABA. At birth, these neurons contain high concentrations of Cl− ions, which means that GABA excites them. But just a week after birth, they contain much less intracellular Cl−, and GABA is inhibitory. The protein responsible for this switch has now been identified, and is shown to be a K+/Cl−co-transporter called KCC2.