The establishment of a model system of neoplastic transformation of normal human cells has been attempted with a chemical carcinogen, 4-nitroquinoline 1-oxide (4NQO). In the course of these experiments, it was noticed that immortalization of human cells is a multi-step process involving several mutational genetic events. Thus, chromosomal changes which occurred during the process of immortalization of human fibroblasts were examined. To accomplish immortalization, fibroblasts obtained from an embryo were repeatedly treated with 10-6M4NQO from primary culture to passage 51 (59 treatments in total). Before immortalization, some chromosomes (especially, chromosomes 2, 6, 8, 10, 11, 12, 15, 19, and 20), were lost at a relatively high frequency. After immortalization, the chromosomes distributed so broadly in the triploid to hypotetraploid region without a distinct modal number or without marker chromosomes that it was difficult to identify the specific chromosomes related to the immortalization of human cells. No specific structural chromosomal changes were detected. Although the significance of such chromosome changes in relation to immortalization is not clear, the loss of some specific chromosomes suggests that genes which are involved in cellular aging and which suppress immortalization may have been lost in the immortalization process.</p
