TIMM21 (translocase of inner mitochondrial membrane 21) is a dual-function mitochondrial protein that operates at the intersection of protein import and respiratory chain assembly. As a component of the TIM23 presequence translocase complex, TIMM21 facilitates translocation of transit peptide-containing proteins across the mitochondrial inner membrane 1. Beyond its canonical import role, TIMM21 functions as a core constituent of the MITRAC (mitochondrial translation regulation assembly intermediate of cytochrome c oxidase complex) complex, where it is required for assembly of respiratory chain complexes I and IV 2. Notably, TIMM21 is dispensable for general protein import but specifically required for integration of presequence-containing subunits into early respiratory chain assembly intermediates, establishing a critical link between mitochondrial protein translocation and respiratory chain biogenesis 2. This function directly impacts mitochondrial translation regulation in response to assembly state of respiratory complexes. Clinically, genetic variation near TIMM21 (rs75769176) has been associated with tuberculosis severity (p = 0.04) in a genome-to-genome study, suggesting its involvement in host-pathogen interactions and disease susceptibility, though the mechanistic basis remains unclear 3.