CFD is becoming increasingly important in the design of gas turbines because correlation based methods are unable to further improve efficiency and laboratory experiments are prohibitively expensive. As first-principles based CFD is too computationally costly in a design context, RANS-based CFD is typically used where turbulence is modelled. However, the inaccuracies introduced by RANS limits the impact CFD can have on technology development. In this presentation, a novel machine-learning based approach is introduced that uses high-fidelity data to improve turbulence closures. It will be shown that closure models developed using the gene-expression programming approach outperform traditional models both for cases they were trained on and for cases not seen before.