Though recognized as prime drug targets, membrane proteins largely remain terra incognito for structural biologists because they are so difficult to express in eukaryotic-cells. Michael Mendez of Gryffin Consulting, a genetic engineering firm, explains that the main difficulty in expressing membrane proteins is cell toxicity: Overexpressed proteins that cannot be processed in the endoplasmic reticulum (ER) initiate apoptosis, killing the cell. Altering the processing mechanism would prevent cell death and allow the cells to churn out more usable protein.