She had Snijders Blok-Campeau syndrome, which is caused by mutations in the CHD3 protein, which manages the structure of DNA. In her case, it was this missense variant: https://www.ncbi.nlm.nih.gov/clinvar/variation/422607/
The severity of Snijders Blok-Campeau syndrome can vary, but it sounds like Mei only suffered from mild intellectual disability.