Vertebrates were once thought to be unable to perform this cycle because there was no evidence of its two key enzymes, isocitrate lyase and malate synthase. However, some research suggests that this pathway may exist in some, if not all, vertebrates. 
 Specifically, some studies show evidence of components of the glyoxylate cycle existing in significant amounts in the liver tissue of chickens. Data such as these support the idea that the cycle could theoretically occur in even the most complex vertebrates. Other experiments have also provided evidence that the cycle is present among certain insect and marine invertebrate species, as well as strong evidence of the cycle's presence in nematode species. However, other experiments refute this claim.  Some publications conflict on the presence of the cycle in mammals: for example, one paper has stated that the glyoxylate cycle is active in hibernating bears, but this report was disputed in a later paper. Evidence exists for malate synthase activity in humans due to a dual functional malate/B-methylmalate synthase of mitochondrial origin called CLYBL expressed in brown fat and kidney. Vitamin D may regulate this pathway in vertebrates.