For many years the etiology of human tumors has almost exclusively been
attributed to environmental factors, which seemed to be involved not
only in the induction of neo- plasms but also in other phases of
tumorigenesis. Cigarette smoking and alcohol are the two best known
examples of factors related to our modern lifestyle which have been
implicated in the development of cancer of various organs. From the
1960s onward, the pioneer work of a few investigators clearly showed
that hereditary factors were not only related to rare neoplasms, but
could also be respon- sible for the occurrence of a sizeable fraction of
tumors that do not usually show features of inheritance (as in the case
of breast or colorectal cancer). Thus, the idea that some common human
malignancies could be transmitted from one generation to another with
features of mendelian gene- tics became more and more evident. In
addition, the verit- able explosion of molecular biology in the last 15
years has made a further and substantial contribution to the emerging
role of genetics in biomedical research and particularly in cancer.