The smallest cancers that we can routinely catch are a billion cells. We all look differently, we behave differently, but our genomes are virtually the same. Where the genome exists in a different environment, it behaves differently. DNA does not exist in a vacuum. It's one thing to treat a disease where a single gene is involved.
Cancer involves many genes and people are different.
It's the nature of a rare disease to teach you the critical piece of information you need to generalize the information.