TDO2 and chronic kidney disease: Thus, it cannot be excluded that KYN may support the early stage of osteoblast differentiation, which is particularly intensified in CKD [32,46], while the inverse relationship between cyclin D1 and TDO gene expression may be an attempt to compensate for this phenomenon, in order to obtain mature osteoblasts (Figure 8) capable of performing their physiological functions, e.g., participation in the mineralization process.