HMOX1 and type 2 diabetes mellitus: In fact, a higher number of (GT)n repetitions in the HMOX1 gene promoter (categorized as class L allele) leads to underexpression of HMOX1 [23] and are of possible clinical significance also in T2DM, as documented by a higher risk of T2DM [25] and a worse clinical outcome [26] in these patients, which were also associated with lower serum bilirubin concentrations [27].