Why are some types of cancer so much more common than others? Sometimes its due to faulty genes inherited from ones parents and sometimes to behaviors like smoking a pack of cigarettes every day. But in most cases, it comes down to something else stem cells.

This is the intriguing argument made by a pair of researchers from Johns Hopkins University. In a study published Friday in the journal Science, they found a very high correlation between the differences in risk for 31 kinds of cancer and the frequency with which different types of stem cells made copies of themselves.

Just how strong was this link? On a scale that goes from 0 (absolutely no correlation) to 1 (exact correlation), biostatistician Cristian Tomasetti and cancer geneticist Bert Vogelstein calculated that it was at least a 0.8. When it comes to cancer, thats high.

No other environmental or inherited factors are known to be correlated in this way across tumor types, Tomasetti and Vogelstein wrote.

Researchers have long recognized that when cells copy themselves, they sometimes make small errors in the billions of chemical letters that make up their DNA. Many of these mistakes are inconsequential, but others can cause cells to grow out of control. That is the beginning of cancer.

The odds of making a copying mistake are believed to be the same for all cells. But some kinds of cells copy themselves much more often than others. Tomasetti and Vogelstein hypothesized that the more frequently a type of cell made copies of itself, the greater the odds that it would develop cancer.

The pair focused on stem cells because of their outsided influence in the body. Stem cells can grow into many kinds of specialized cells, so if they contain damaged DNA, those mistakes can spread quickly.

The researchers combed through the scientific literature and found studies that described the frequency of stem cell division for 31 different tissue types. Then they used data from the National Cancer Institutes Surveillance, Epidemiology and End Results database to assess the lifetime cancer risk for each of those tissue types. When they plotted the total number of stem cell divisions against the lifetime cancer risk for each tissue, the result was 31 points clustered pretty tightly along a line.

To put this notion in concrete terms, consider the skin. The outermost layer of the skin is the epidermis, and the innermost layer of the epidermis contains a few types of cells. Basal epidermal cells are the ones that copy themselves frequently, with new cells pushing older ones to the skins surface. Melanocytes are charged with making melanin, the pigment that protects the skin from the suns damaging ultraviolet rays.

When sunlight hits bare skin, both basal epidermal cells and melanocytes get the same exposure to UV. But basal cell carcinoma is far more common than melanoma about 2.8 million Americans are diagnosed with basal cell carcinoma each year, compared with roughly 76,000 new cases of melanoma, according to the Skin Cancer Foundation. A major reason for this discrepancy, Tomasetti and Vogelstein wrote, is that epidermal stem cells divide once every 48 days, while melanocytes divide only once every 147 days.

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Scientists explain how stem cells and 'bad luck' cause cancer