During oogenesis, Xenopus oocytes accumulate large amounts of storage proteins that—after fertilization—provide the developing embryo with building blocks and energy metabolites. The size of the fully developed oocyte (diameter, 1.2 mm) is largely governed by the stored amounts of egg yolk protein. Thus, the mature oocyte is equipped to initiate protein synthesis, cell growth, and replication after fertilization. The size of the oocyte, the large reserve of storage proteins, and its ability to synthesize protein on demand makes the oocyte an almost ideal single-cell expression system. Some basic physical properties of this expression system are listed in Table 1.