CBS and cancer: However, in plasma most proteins are reversibly post-translationally modified by cysteinylation114 and, as mentioned above, s-cysteinylated albumin represents a major source of cysteine for cells, including cancer cells.13 Although the cellular pathways required for protein s-cysteinylation are not fully elucidated (Fig. 4), evidence shows that CBS might have a relevant role in this setting, as no s-cysteinylated albumin was detected in CBS-deficient mice.200