CYCS and mucopolysaccharidosis type 4A: This analysis confirmed that there are many apoptosis-related genes in which expression is dysregulated in MPS IVA and IVB cells, including some playing crucial roles in this process, like CYCS (coding for cytochrome c), PDK1 (coding for pyruvate dehydrogenase kinase 1), and XIAP (coding for X-linked inhibitor of apoptosis) (Table 1).