BCS1L and Björnstad syndrome: Similarly, the assembly and function of human mitochondrial complex III depends on the BCS1L protein, and mutations of BCS1L can cause Björnstad syndrome (Bénit et al. 2009), which is a combination of sensorineural hearing loss and pili torti, a condition in which the hair shafts are thin and broad, and twisted along their long axis (Rogers 1995).