Abstract
Characterization of C-type RNA viruses suggested that genetic and environmental factors are involved in the expression of the leukemia viruses. A reverse transcriptase in C-type viruses, in human and animal leukemic leukocytes provides a mechanism by which an RNA tumoral virus may transform permanently a normal cell. However, it has been found the presence of reverse transcriptase in normal and different hematological conditions. The study of the genome of human leukemic cells is also related with the characterization of satellite DNAs, which are families of nucleotide sequences having different repetition frequencies. Experimental evidence obtained in animals make it possible that satellite DNA might be a point of insertion of viral gene in the cell to be transformed.