Primary cell cultures obtained directly from normal tissue samples can only divide a limited number of times. To evade this problem, scientists chemically or genetically manipulate cells to create cell lines—immortalized cells that can proliferate indefinitely. Cell lines can also be propagated directly from cancerous cells. Besides immortality, such cell lines have higher growth rates and can continue to grow even when surrounded by other cells or not attached to a solid surface. Primary cell lines can acquire spontaneous mutations in cancer-associated genes or be deliberately exposed to cancer-inducing viruses or chemicals, creating cancerous cell lines. Over time, cell lines can undergo phenotypic and genetic changes. They are also prone to cross-contamination in the lab. Despite their limitations, they are easy to handle and cost-effective and bypass many ethical issues associated with animal research. Therefore, labs routinely use cell lines to model diseases, test drugs, and mass-produce antibodies and vaccines.