TBCD and Onset: More recently, homozygous mutations in TBCD, a gene encoding one of the five tubulin-specific chaperones that are required for α/β-tubulin de novo heterodimer formation, have been described in association with an infantile onset neurodegenerative disorder characterized by developmental regression, seizures, optic atrophy and secondary microcephaly, cortical atrophy with delayed myelination, cerebellar atrophy and thinned corpus callosum.