Before revision:  The reason why cancer is so difficult to treat is because CACC is flexible, which enables cancer cells not only to produce various heterogeneous subclones through limited cell differentiations , but also to maximally and efficiently use genome related resources to survive environmental changes.
After revision:  The reason why cancer is so difficult to treat is because CACC is flexible, which enables cancer cells not only to produce various heterogeneous subclones through limited cell differentiation , but also to maximally and efficiently use genome related resources to survive environmental changes.