[Malignant melanoma as cancer model].

Malignant melanoma is a suitable model for studying the growth of malignant tumors in general, because there are no obstacles to observation during its course and it can easily be obtained. Local progression of the tumor implies stepwise changes from low to high malignancy. However, regression can also be recognized, often simultaneously with progression. Both phenomena can be explained with reference to heterogeneity of tumor cells and instability of phenotypes. These are reflected not only in morphologic differences but also in the production of enzymes and factors and the expression of antigens. Phenotypic dynamics are not haphazard but rather according to definite principles. Levels of early tumor-associated and HLA-A,B,C (class I) antigens decline with advancing progression, whereas those of some late "risk" antigens, including HLA-D (class II) antigens, increase. Tumor and host interact, apparently in alternating directions, until biologic preponderance is established.