Abstract
A substantial part of the mammalian genome is composed of sequences that do not contain structural genes. These sequences may constitute the major target for physical, chemical and biological DNA-damaging agents and can be involved in carcinogenesis. DNA-damaging agents contribute to the instability of the genome by introducing recombination-prone sites at DNA; these agents lead to extensive chromosomal lesions and rearrangements of genes and their regulatory sequences. Movable sequences that exist and operate in certain bacteria, yeast, and the fruit fly are responsible for sequence rearrangements and contribute to the majority of mutations. Their presence and role in higher animals is not well established. Extensive chromosomal rearrangements were identified in numerous malignancies in man and animals and definitely seem to represent a characteristic of malignancy. Vast chromosomal damage and sequence reshuffling may be of no less importance in the malignant transformation than the point mutation of a particular gene.