A checkpoint is a control point where progress is paused until certain conditions are met. The term appears across many contexts, from airport security to video game save points, but its most significant scientific meanings come from biology and medicine. Inside your body, checkpoints govern two critical processes: how cells divide and how your immune system decides what to attack. Understanding both helps explain some of the most important advances in cancer treatment over the past two decades.
Checkpoints in Cell Division
Every cell in your body follows a sequence of steps when it divides, known as the cell cycle. Built into this sequence are three major checkpoints, moments where the cell essentially asks, “Is everything ready to move forward?” If the answer is no, the cell halts until the problem is fixed or, in some cases, destroys itself entirely. These checkpoints exist to prevent damaged or abnormal cells from multiplying, which is one of the body’s primary defenses against cancer.
The molecular engines driving cell division are proteins called cyclin-dependent kinases. These enzymes are always present in the cell but remain inactive until partner proteins called cyclins bind to them at the right time. Think of it like a car with the engine always installed but the ignition key only handed over at specific moments. Additional layers of control can block these enzymes even after activation, giving the cell multiple chances to stop the process if something goes wrong.
The G1 Checkpoint (Restriction Point)
The first major checkpoint occurs before a cell commits to copying its DNA. Known as the restriction point, this is where the cell evaluates whether conditions are favorable for division. It checks for adequate nutrition, sufficient size, and the presence of external growth signals from surrounding tissue. The restriction point sits roughly two to three hours before DNA copying begins.
Growth factors from neighboring cells are essential for pushing through this checkpoint. If those signals disappear before the cell reaches the restriction point, division stops. But once the cell passes this threshold, it no longer needs external permission. It becomes committed to completing division regardless of whether growth signals continue. This “point of no return” quality makes it one of the most important gatekeepers in biology. When this checkpoint malfunctions, cells can begin dividing without the normal growth signals, a hallmark of cancer.
The G2 Checkpoint
After the cell copies its DNA, a second checkpoint verifies that the copying happened correctly. DNA replication is an enormously complex process, and errors inevitably occur. The G2 checkpoint scans for damage or incomplete replication and blocks the cell from entering the physical division stage until repairs are made. Specialized sensor proteins detect problems and trigger a cascade of signals that halt the cycle. The cell also reassesses its size at this point, ensuring it has grown enough to successfully split into two functional daughter cells.
The Spindle Checkpoint
The final checkpoint occurs during the physical act of division, right before the cell pulls its copied chromosomes apart. At this stage, each chromosome must be properly attached to the molecular cables (called spindle fibers) that will tug them to opposite sides of the cell. The checkpoint works through a beautifully direct mechanism: any chromosome that isn’t properly attached sends out a “wait” signal. Specifically, unattached connection points on chromosomes accumulate proteins that block the cell from proceeding. Only when every single chromosome is correctly connected and under mechanical tension does the signal stop, allowing the cell to split.
This checkpoint is remarkably sensitive. Even one unattached chromosome out of 46 is enough to halt the entire process. The physical pull on the connection points changes their chemistry, shutting off the wait signal at each individual chromosome independently. Once all signals are silenced, the cell finally proceeds.
Immune Checkpoints
Your immune system uses a completely different kind of checkpoint. T cells, the immune system’s primary killers, go through a selection process in the thymus gland where cells that react too strongly to the body’s own tissues are eliminated. But this screening isn’t perfect, so the body has additional brakes built into T cell behavior called immune checkpoints. These are protein interactions on the surface of T cells that can dial down or shut off an immune attack.
Two immune checkpoints are especially important. The first, called CTLA-4, acts early in the process when T cells are first being activated in lymph nodes. CTLA-4 competes with an activating protein for the same binding partner. When CTLA-4 wins that competition, the T cell receives a “stand down” signal instead of an “attack” signal. The balance between these competing proteins determines whether a T cell gets activated or goes quiet. Regulatory T cells, which specialize in suppressing immune responses, permanently display CTLA-4 on their surface.
The second checkpoint, PD-1, works later and in a different location. PD-1 appears on T cells that are already active and engaged in an immune response out in the body’s tissues. When PD-1 encounters its partner protein (PD-L1) on a nearby cell, it reduces the intensity of the T cell’s attack. This mechanism protects healthy tissues from collateral damage during an immune response. Because PD-1 acts on a narrower set of already-active T cells, it produces a more targeted form of immune suppression than CTLA-4.
How Cancer Exploits Immune Checkpoints
Cancer cells face a problem: the immune system can recognize them as abnormal and try to destroy them. Some tumors solve this by producing large amounts of PD-L1 on their surface. When a T cell arrives ready to attack, the tumor’s PD-L1 binds to PD-1 on the T cell and sends it an “off” signal. The T cell stands down, and the tumor survives. In effect, the cancer hijacks a safety mechanism designed to protect healthy tissue and uses it as a shield.
Checkpoint Inhibitors in Cancer Treatment
The discovery that blocking these immune checkpoints could unleash the immune system against cancer earned James P. Allison and Tasuku Honjo the 2018 Nobel Prize in Physiology or Medicine. In 1994, Allison’s lab at UC Berkeley treated tumor-bearing mice with antibodies that blocked CTLA-4. The results were striking: even pre-established tumors shrank, and the mice developed lasting immunity against those cancers. Meanwhile, Honjo’s group in Kyoto had identified PD-1 and demonstrated that it also served as a brake on anti-cancer immune responses.
These findings led to a class of drugs called immune checkpoint inhibitors. Rather than attacking cancer directly like chemotherapy, these drugs remove the brakes on the immune system, allowing T cells to recognize and destroy tumor cells. There are now 11 FDA-approved checkpoint inhibitors, including pembrolizumab (Keytruda) and nivolumab (Opdivo), approved across numerous cancer types.
The survival improvements have been substantial, particularly in advanced melanoma, which was once considered nearly untreatable. In clinical trials, patients with advanced melanoma who received a combination of two checkpoint inhibitors had a 52% five-year survival rate, compared to 26% for those receiving a single older checkpoint inhibitor. For patients whose tumors had certain genetic mutations, the five-year survival rate reached 60% with combination therapy.
Side Effects of Releasing Immune Brakes
Because checkpoint inhibitors work by removing natural restraints on the immune system, they can cause the immune system to attack healthy tissues. These are called immune-related adverse events. In clinical studies, lung inflammation occurred in roughly 12.5% of patients, skin reactions in about 8.8%, and thyroid problems in 6.3%. The specific rates vary depending on which drug is used and whether drugs are combined. These side effects reflect the fundamental biology: the same checkpoints that cancer exploits to hide also protect your organs from immune attack. Removing those brakes helps fight the tumor but can leave healthy tissue exposed.