Objective: To identify the underlying genetic defect in a patient with intellectual disability, seizures, ataxia, macrothrombocytopenia, renal and cardiac involvement, and abnormal protein glycosylation. Methods: Genetic studies involved homozygosity mapping by 250K single nucleotide polymorphism array and SLC35A1 sequencing. Functional studies included biochemical assays for N-glycosylation and mucin-type O-glycosylation and SLC35A1-encoded cytidine 59-monophosphosialic acid (CMP– sialic acid) transport after heterologous expression in yeast. Results: We performed biochemical analysis and found combined N- and O-glycosylation abnor- malities and specific reduction in sialylation in this patient. Homozygosity mapping revealed homozygosity for the CMP–sialic acid transporter SLC35A1. Mutation analysis identified a homozygous c.303G.C (p.Gln101His) missense mutation that was heterozygous in both pa- rents. Functional analysis of mutant SLC35A1 showed normal Golgi localization but 50% reduction in transport activity of CMP-sialic acid in vitro. Conclusion: We confirm an autosomal recessive, generalized sialylation defect due to mutations in SLC35A1. The primary neurologic presentation consisting of ataxia, intellectual disability, and seizures, in combination with bleeding diathesis and proteinuria, is discriminative from a previous case described with deficient sialic acid transporter. Our study underlines the importance of sialylation for normal CNS development and regular organ function. Neurology" 2013;81:1–7