An insight into the ordered pattern of enzymatic and biochemical imbalance of cancer cells was made possible, at least in part, by the molecular correlation concept, the concept of key enzymes, and the use of biological model systems. With these approaches the pattern of gene expression in neoplasia can be studied in terms of the activities of key enzymes and their linking with neoplastic transformation and progression. An ordered pattern of biochemical imbalance was recognized in carbohydrate, pentose phosphate, purine, pyrimidine, DNA, and polyamine metabolism and in other biochemical areas. Current work is directed to clarifying the enzymology and metabolic pattern of thymidine metabolism and CTP biosynthesis since these areas are of particular importance in selective chemotherapy and rescue. The activities of key enzymes in thymidine metabolism have been correlated with the growth rates in a spectrum of hepatomas. Increases in the activities of four key enzymes in CTP biosynthesis appear to be specific to neoplasia because no similar pattern was observed in the normal adult resting or regenerating liver or in the fetal and developing liver. The overall pattern discovered in transplantable rat hepatomas applies to other rodent tumors. It is of particular importance that the pattern of biochemical imbalance is also applicable to human hepatocellular carcinomas. With the recognition of the ordered pattern of reprogramming of gene expression in hepatomas, the path is open for the development of sensitive assays for biochemical diagnosis of liver tumors and for a rational design of selective chemotherapy and rescue.