NRF1 and obesity due to melanocortin 4 receptor deficiency: Obesity was also associated with a reduced mRNA expression of the mitochondrial transcription factor A (Tfam) and the nuclear respiratory factor-1 (NRF1), which are two key regulators for mitochondrial biogenesis, and with strongly reduced protein and mRNA expressions of NADH dehydrogenase 6 (ND6), a subunit of complex I in the respiratory chain [10].