These molecular defects cause abnormal functions of the cartilage extracellular matrix (COMP), paracrine signalling factors (PHEX, FGFR3-related rickets), transcriptional regulation (LZTR1), histone methylation (KMT2A), posttranslational modification (NAA15), and mtDNA replication and repair (POLG) (Andrade et al., 2017). The gene discussed is POLG; the disease is rickets.