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根据以下材料回答第 1～4 题：Passage One
Since the early l980s，scientists have revealed some 40 human genes involved in caneer．These genes are essential for normal growth，but can be subverted to cause a tumor．
Dr．Jorge Yunfs of the University of Minnesota Medical School in Minneapolis hasfound that 70 percent of oncogenes，or cancer-causing genes，are located near inheritedweak points on chromosomes(染色体)．Varying from individual to individual，vulnerableto chemical carcinogens(致癌剂)，X rays and other cancer_inducing agents．
“If a chromosome snaps apart in the immediate vicintiy of an oncogene，”says Yunis，“normal genetic control mechanisms could break down and the stage would be set for the formation of cancer．”Younis has shown that such a sequence OCCURS at the beginning of numerous leukemias(白血病)，lymphomas(淋巴瘤)and some tumors of the lun9，coIon(结肠)and breast．
Yunl‘s and other inverstigators have found that petroleum-based products-notably Desticides and insecticidesdamage specific sites on at least two of the 23 pairs of human chromosomes that carry genetic information．Similarly，tobacco smoke tends to attack a part of another ehromosome．
From paragraph I，we know that some 40 genes involving in cancer are ．
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