These new cell lines were then tested rigorously to make sure that the genetic code of the cell lines remained stable as they replicated and retained the same characteristics as primary cells. To make sure the immortalized cells hadn’t developed cancer-like characteristics, the scientists looked for any chromosomal abnormalities and tried to grow both the new lines and a line of cancer cells on soft agar — only cancer cells should be able to grow on this medium. The cell lines displayed no chromosomal abnormalities and couldn’t grow on the agar. The scientists also confirmed that the cell lines behaved like their unmodified primary counterparts by testing their protein and mRNA production.