Protein misfolding toxicity is a major factor in neurological diseases like Alzheimer’s and ALS. The high load of misfolded proteins found in tumors has also been called cancer’s ‘Achilles heel.’ But to understand how misfolding impacts human health, we need to quantify how cell fitness declines as misfolded proteins accumulate. A reigning hypothesis as to why misfolded proteins are toxic is that they steal important resources (eg, chaperones). This implies that the toxicity of a misfolded protein depends on the number of resources available. Our goal is to make accurate predictions about the context-dependent costs of misfolding mutations by quantifying the relationship between the number of misfolded proteins in a cell and cell fitness. Deciphering this relationship will improve predictions about the combined costs of multiple misfolding mutations. It will also reveal when and how these extremely common, but typically small-effect, mutations contribute to evolutionary processes.