TFAM and colorectal carcinoma: Furthermore, a site-directed mutagenesis assay of serine residues indicated that phosphorylation of TFAM was inhibited when serine-55 was mutated to alanine in MCU-knockdown CRC cells, whereas the phosphorylation state of TFAM was not affected when serine-160 or −170 was mutated to alanine (Fig. 6c), indicating that serine-55 of TFAM was the primary phosphorylation site.