KCC2b (Slc12a5) global knockout heterozygous mice on a C57BL/6J background. The Slc12a5 gene has alternative promoters which produce isoforms starting with exon 1a (5-10% total expression) and exon 1b (90-95% expression). As we targeted exon 1b, the mouse in not a global KO of KCC2 but a hypomorphic mouse, still expressing 5-10% protein. It is a global KCC2b KO mouse. Heterozygous animals develop spontaneous seizures in cages. This increases with age. Their brain is hyperexcitable. Homozygous animals die between P17 due to repeated tonic-cloning seizures.