Producing recombinant mammalian proteins in native or near-native conformation is fundamental to many aspects of biology. Unfortunately, it is also a task whose outcome is extremely unpredictable. A protein that has been shaped over millions of generations of evolution for expression at a level appropriate to a specific cell type or in a particular developmental stage, may be toxic to a new host cell, or become insoluble (among many possible obstacles) when overexpressed in vitro. The object of this volume, "Protein Expression in Mammalian Cells," is to offer guidance for those who wish (or who have been forced by circumstance) to overexpress a mammalian protein in mammalian cells.