Final answer:
Density-dependent inhibition is where cell division slows down or stops when cells reach a high density, something cancer cells often ignore, leading to uncontrolled growth and tumor formation.
Step-by-step explanation:
Density-dependent inhibition is a phenomenon observed in normal cells, where the proliferation of the cells is regulated based on the density of the cell population. Cells sense how many other cells are around by direct cell-to-cell contact; when there are enough cells to form a complete layer, they stop dividing. However, cancer cells often lose this ability. They do not properly respond to the signals that regulate the cell cycle and continue to proliferate even when surrounded by a dense population of cells. This uncontrolled division leads to tumor formation and is a characteristic behavior of cancer cells.