HAL and histidinemia: While methane, nitro- has not been detected in human blood, excretions such as urine and feces, or saliva, rats subcutaneously injected with methane, nitro- were used as a model of human histidinemia, showing increased levels of histidine in blood, urine, and cerebrospinal fluid, in which methane, nitro- is a histidine ammonia-lyase inhibitor (HAL)42.