Isocitrate dehydrogenase (ICD) is a crucial enzyme in the TCA cycle. Reversible inactivation of the enzyme by phosphorylation at Ser113 regulates metabolic switching from the TCA cycle to the glyoxylate pathway, which is adopted by bacteria, fungi and plants in the absence of a \"complex\" carbon source such as glucose. In humans, specific mutations within ICD are found in several forms of brain tumour, with some specificity between pathologies. Mutations in ICD are detected in almost all forms of secondary glioblastoma which form from lower-grade glioma, but they are rarely found in primary high-grade glioblastoma multiforme. A correlation between the presence of a ICD mutation and increased life expectancy from secondary glioblastoma has been suggested [PMID:19636000].