Because the loss of the Smad4 gene could promote the initiation, development, migration, and invasion of CRC [26,27,28], and due to the high expression of Smad4 gene in HCT116 cells (Figure 2C) and the strongest cytotoxic effect of CD55-Smad4 on HCT116 cells (Figure 2B), for further experiments, the Smad4 gene was knocked down in HCT116 cells (HCT116-Smad4−/−) (Figure 2D). Here, CD55 is linked to colorectal carcinoma.