Modification of this approach to assess acute hormonal activation of each isoenzyme in cells in culture revealed that selective isoenzyme activation of individual isoenzymes I and II can occur in osteoblasts and osteosarcoma cells in response to PTH (43, 67), as well as in human breast cancer cells T47D and MCF7 in response to calcitonin and PGE2 (68). This evidence concerns the gene PTH and osteosarcoma.