Up to now, protein engineering attempts have been limited by the need for purification of each single variant prior to bioactivity tests in cell culture, during which product loss or deactivation can easily occur. acib’s purification-free approach based on the high throughput monitoring of mammalian cell proliferation does not share this limitation. Skipping purification not only spares time and money, allowing direct assessment of biological activity of the mutant variants, but also enables screening of bigger libraries, greatly facilitating protein engineering of growth factors for higher potency and stability against various stressors – temperature, pH, proteases. Higher stability also means greater expression yields, and competitive prices. This system is compatible with both classic and de-novo protein engineering approaches.