Final answer:
The most important property of cancer cells is their ability to divide and grow uncontrollably, forming multiple layers of cells. This is due to deficiencies in cancer cells, such as the inability to form gap junctions and adhere firmly to an extracellular matrix.
Step-by-step explanation:
The most important property of cancer cells both in vivo and in vitro is their ability to divide and grow uncontrollably. Unlike normal cells, cancer cells do not stop dividing when they reach confluence, leading to the formation of multiple layers of cells. This uncontrolled growth is a result of various deficiencies in cancer cells, such as the inability to form gap junctions and adhere firmly to an extracellular matrix. These properties contribute to the invasiveness and metastasis of cancer cells.