There are various immortal cell lines. Some of them are normal cell lines (e.g. derived from stem cells). Other immortalised cell lines are the in vitro equivalent of cancerous cells. Cancer occurs when a somatic cell which normally cannot divide undergoes mutations which cause de-regulation of the normal cell cycle controls leading to uncontrolled proliferation. Immortalised cell lines have undergone similar mutations allowing a cell type which would normally not be able to divide to be proliferated in vitro. The origins of some immortal cell lines, for example HeLa human cells, are from naturally occurring cancers.