CYB5R3 and methemoglobinemia: Naturally occurring CYB5R3 deficiency causing methemoglobinemia (equivalent to type I RCM in humans) has also been reported in dogs (Online Mendelian Inheritance in Animals [OMIA] #002131-9615, [14,15,16,17,18,19,20,21]) and cats (OMIA #002131-9685; #01171-9685, [14,15,22,23,24,25]).