These microglial “replacements” have been successful in alleviating the neurological and systemic symptoms in MECP2 mutant mice, a model for Rett syndrome (Derecki et al., 2012), in correcting abnormal grooming behavior in Hoxb8 mutant mice, a possible model for obsessive-compulsive disorder (Chen et al., 2010), and in increasing survival in SOD1G93A transgenic mice, a model for amyotrophic lateral sclerosis (ALS) (Lee et al., 2012b). The gene discussed is MECP2; the disease is amyotrophic lateral sclerosis.