Substances that cause DNA mutations are known as mutagens, and mutagens that cause cancers are known as carcinogens. Particular substances have been linked to specific types of cancer. Tobacco smoking is associated with many forms of cancer, and causes 90% of lung cancer.
Human diploid cells have 46 chromosomes and human haploid gametes (egg and sperm) have 23 chromosomes. Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid.
The normal diploid number of chromosomes is constant for each species. For humans, the diploid number is 46 (23 pairs in all somatic cells). In the formation of gametes (ovum and spermatozoon), the number is reduced to one half (haploid number); i.e., the ovum and sperm each contain 23 chromosomes. Of these, 22 are autosomes and one is the sex chromosome (X or Y).