Many proteins in cancer cells exhibit increased or decreased expression compared to their levels in normal cells. Some of these proteins, i.e., those encoded by oncogenes and tumor suppressor genes, play key roles in tumorigenesis and in the development of metastases, while others, most likely including those involved in intracellular Ca2+ homeostasis (reviewed in Refs. 1 and 2), are associated with cancer progression but are not causative in further development of the tumor and/or malignant cells (reviewed in Ref. 3). Most cancers are heterogeneous with respect to rates of growth and degrees of aggression. This most likely reflects the fact that, for a given cancer, there can be different combinations of oncogenes and tumor suppressor genes that are mutated, different sequences in which these mutations occur, and variations in the time over which the mutations accumulate.
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