TTC19 (tetratricopeptide repeat domain 19) is a mitochondrial protein essential for maintaining the structural and functional integrity of respiratory complex III 12. It facilitates physiological turnover of the Rieske iron-sulfur protein UQCRFS1 and clears N-terminal fragments generated during UQCRFS1 incorporation into complex III; these fragments impair catalytic activity if not removed 2. TTC19 functions as a complex III assembly factor 3. Biallelic TTC19 mutations cause mitochondrial complex III deficiency, nuclear type 2 (CIII deficiency type 2), an autosomal recessive disorder 4. All reported TTC19 mutations are nonsense or frameshift variants 3. Clinical manifestations include progressive neurodegeneration with Leigh-like brain MRI abnormalities, cerebellar ataxia, spastic paraplegia, cognitive dysfunction, and developmental delay, with variable age of onset and disease progression 356. Emerging phenotypes include palatal tremor, hypertrophic olivary degeneration, and axonal neuropathy 6. Genetically predicted lower TTC19 expression is associated with increased Parkinson's disease risk 78. Disease severity does not correlate with residual complex III enzyme activity 3, suggesting variable expressivity among patients with similar mutations.