TTK and cancer: TTK used in the clinical study is considered to be very appropriate because it was expressed in the great majority (> 95%) of esophageal cancers, was expressed specifically in cancer cells and testis (cancer–testis antigens), was shown to be essential for the survival of cancer cells (Mizukami et al. 2008), and most importantly revealed very strong immunogenicity (Suda et al. 2007; Kono et al. 2009, 2012).