Final answer:
A mass of cells originating from a mutant parental line and remaining localized is known as a benign tumor. Benign tumors can grow but don't metastasize; if they do, they become malignant cancers.
Step-by-step explanation:
A mass of cells derived from a mutant parental line, but that has stayed at the site of origin, could be called a benign tumor. Benign tumors are masses of cells that continue to grow, but remain localized and do not invade other tissues or spread to other parts of the body. These can include various types of non-cancerous growths like fibroids, moles, and even immortal cell lines in a laboratory setting. However, if a benign tumor gains the ability to metastasize, it becomes malignant and is then categorized as cancer. In the context of cell lines, continuous lines derived from transformed cells or tumors that can grow indefinitely are a testament to the resilience and complexity of such cellular systems.