HeLa cells are a line of human cells derived from cervical cancer. They have shown high durability and versatility in research, and have been used for applications such as drug discovery, study of viruses, and cancer research. Due to evolution of the line, which was created in 1951, many strains of HeLa cells are now available. Some varieties contain a tetracycline repressor protein, papilloma virus sequences, or Chang liver cells derived from tissues contaminated with HeLa. Many HeLa cell strains have specific features and characteristics like the presence of keratin and resistance to polioviruses. Quality and thorough documentation are important when investing in HeLa cells.