Genetic and sequencing studies have identified many genes which increase risk or predispose to getting cancer. In contrast, there is little known about which genes could harbour alterations which could act protectively and reduce this risk. Our transformative idea is to harness the power of Mendelian (single gene) disorders - the strongest mutations observed in humans - to identify genes linked to reduced growth (height or brain size) which could act to slow growth in cancer. We will utilise CRISPR technology in a high throughput screening approach to test the relevance of these genes to slowing growth in cancer models and their possible candidacy as protective genes. Our novel approach will identify new candidate genes to consider in cancer risk, and has the potential to enable personalised risk scores, for strong clinical benefit in higher risk individuals, as well as novel therapeutic targets.