Cell division is a normal phenomenon for all cells as this is the way the cell reproduces and gets repaired in living organisms. However, some cells, due to mutation, keeps dividing and proliferating to form tumours. These cells are called CANCER cells. A normal cell undergoes cellular repair at certain checkpoints of the cell cycle. The checkpoints are necessary to determine a faulty cell and stop its division.
However, cancerous cells do not undergo any repair, which is why they do not enter the G0 phase as mentioned in the question. They keep on dividing out of control without death by repeating the cell cycle continuously.
