The study focuses on Timothy syndrome (TS), a severe disorder characterized by various neuropsychiatric conditions. TS type 1 (TS1) is linked to a genetic variant in the CACNA1C gene’s exon 8A. Previous research revealed several neuronal phenotypes associated with TS1, including abnormal calcium signaling and impaired neuronal migration. The study aimed to develop a potential therapeutic strategy by targeting exon 8A using antisense oligonucleotides (ASOs). They successfully decreased exon 8A inclusion both in vitro and in vivo, leading to significant improvements in patient-derived cortical organoids and forebrain assembloids. Transplantation of ASO-treated cells also rescued calcium changes and dendrite retraction in patient neurons. This demonstrates the potential of suppressing CACNA1C exon 8A expression as a treatment for TS1, highlighting the effectiveness of a multilevel stem cell-based approach in identifying therapeutic strategies for neurological disorders.