Since St3gal3 null mutant mice display severe developmental delay, neurological deficits, including spontaneous seizures, and overall reduced viability, thus limiting behavioral and neurobiological assessment, we here investigated the effects of partial inactivation of St3gal3 in male and female heterozygous knockout (St3gal3±) mice with focus on behavior as well as expression of markers linked to sialylation and myelination pathways. The gene discussed is ST3GAL3; the disease is Global developmental delay.