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This modification can occur either spontaneously during cellular division, or under the influence of external agents called mutagens (physical, chemical or biological agents affecting the structure or number of genes or chromosomes). A gene modified in this way is transmitted to daughter cells. Certain mutations have no consequence on the cell. Others constitute the first step in a long process of carcinogenesis. Thus cancer is considered as a genetic disease since it starts at the scale of a gene inside a cell. Nonetheless, cancer is not a hereditary disease.