The petite is characterized by a deficiency in cytochromes (a, a3 + b) and a lack of respiratory enzymes which engage in respiration in mitochondria. Due to the error in the respiratory chain pathway, petite yeast is incapable of growing on media containing only non-fermentable carbon sources (such as glycerol or ethanol) and forming small colonies when grown in the presence of fermentable carbon sources (such as glucose). The absence of mitochondria can cause the petite phenotype, or by deletion mutations in mitochondrial DNA (termed "cytoplasmic Petites") which is a deletion mutation, or by mutations in nuclear-encoded genes involved in oxidative phosphorylation.