Regression, understood as the loss of previously acquired behaviors, affects 20-30% of patients with Autism Spectrum Disorder (ASD). Nowadays, the mechanism behind this regressive process is still unknown. To improve our knowledge and find selected target treatments, it is important to establish a model with regressive features. Here, we showed that the Synapsin II Knockout (SynII KO) mouse loses previously acquired social and cognitive abilities in adulthood. Synapsin II is a presynaptic phosphoprotein belonging to the Synapsin family (Syns) involved in neuronal development and in the modulation of neurotransmitter release at presynaptic terminals. Mutations in the SYN2 gene are associated with ASD and epilepsy, and in line, SynII KO mice exhibit an ASD-like behavior and develop generalized seizures starting from 2-3 months of age. Here, we report that 1-month-old SynII KO mice show no defects in social and cognitive behaviors, which instead are manifested in adult and epileptic mice, in association with a disrupted sensory and multisensory integration. The regressive manifestations were not related to epilepsy and synaptic alterations appeared with a different timing in the diverse brain regions. Our data suggest the existence of an ASD-genesis latent period during which plasticity mechanisms contrast the precocious excitatory/inhibitory (E/I) imbalance and thus mask the behavioral deficits. Overall, this study proposes the SynII KO mouse as an ideal candidate for the study of ASD with regressive pattern.The research project was funded by the #NEXTGENERATIONEU project MNESYS PE000000, Curiosity Driven 100008-2022 and PRIN PNRR P202299E48.