TMEM186 is a transmembrane protein localized to the mitochondrial inner membrane that functions as a component of the mitochondrial complex I intermediate assembly (MCIA) complex. As part of the MCIA complex alongside NDUFAF1, ECSIT, ACAD9, and TMEM126B, TMEM186 is required for efficient assembly of mitochondrial respiratory chain complex I 1. The protein enriches with newly translated ND3 subunit during complex I biogenesis and is essential for the assembly and integration of the ND2-module, a distinct intermediate in the stepwise construction of complex I 1. Loss of TMEM186 results in defects of the MCIA complex and reduced overall complex I assembly 1. TMEM186 belongs to the TMEM70/TMEM186/TMEM223 protein family, which is evolutionarily conserved only in species possessing oxidative phosphorylation (OXPHOS) complexes, suggesting a specialized role in mitochondrial bioenergetics 2. By supporting complex I assembly, TMEM186 contributes to proper mitochondrial function and oxidative energy production, though specific disease associations remain to be established.