Williams syndrome which exhibited unique neuropsychological profile including partial autistic-like behavioral property is caused by hemizygous deletion of 7q11.23. This region contains the gene for HPC-1/syntaxin1A (STX1A) which is believed to regulate synaptic transmission. Previously, we have produced STX1A knockout mice. Interestingly, consolidation and extinction of conditioned fear memory was impaired in homozygote (KO), but was normal in heterozygote (HT). In this study, we have examined if STX1A knockout mice exhibited abnormal behavior. KO mice exhibited almost normal learning properties in a discriminated operant task, unlike consolidation of conditioned fear memory test. However, latent inhibition of cued fear memory (LI) was attenuated both in HT and KO mice. The attenuation of LI in these mutant mice was recovered by administration of SSRI, Fluoxetine, implying serotonergic disturbance. In novel object exploration test, HT and KO mice explore the familial object for longer time than WT mice. Behavioral abnormality in HT and KO mice were also observed in social interaction test. These results suggested that haploinsufficiency of STX1A might cause for unique neuropsychological profile in WS. In addition, STX1A knockout mice revealed autistic-behavior and could be a model animal of autism. [J Physiol Sci. 2007;57 Suppl:S134]