Other cell lines were in use before 3t3 cells, but the newer culture line soared to the forefront of science because of its particular characteristics. Unlike other tissue cultures available at the time, 3t3 cells did not cause tumors when injected in mice. Careful study led to the discovery of the underlying cause, which is that the cells exhibit a highly developed inhibition of growth based on density levels. In fact, the behavior of 3t3 cells established an unambiguous distinction between the capacity to grow indefinitely and the ability to develop tumors.