Final answer:
The negative regulator proteins of the cell cycle were discovered in cells that had become cancerous due to faulty copies of these proteins. This includes retinoblastoma protein (Rb), p53, and p21.
Step-by-step explanation:
The negative regulator proteins of the cell cycle were discovered in various types of cells, but the best understood ones, such as retinoblastoma protein (Rb), p53, and p21, were discovered in cells that had become cancerous. These proteins, which act as tumor suppressors, were found to be damaged or non-functional in cells that had lost regulatory control and started replicating uncontrollably. The main cause of the unchecked progress through the cell cycle in these cells was a faulty copy of the regulatory protein.
Many of the negative regulator proteins of the cell cycle were discovered in cells that had become cancerous. Negative regulators such as retinoblastoma protein (Rb), p53, and p21 play a crucial role in controlling cell division by arresting the cell cycle until certain conditions are met. These proteins are especially important in preventing uncontrolled cell division, which is characteristic of cancer. The damaged or non-functional versions of these proteins were discovered in cancerous cells that had begun to replicate uncontrollably, due to faulty copies of the regulatory proteins.