Hallmarks of cancer—Then and now, and beyond

For over 25 years, the "hallmarks of cancer" have served as a conceptual framework to organize the immense complexity of cancer biology. Originally proposed to distill the diverse genetic and phenotypic traits of tumors into a manageable set of principles, the framework has expanded from six initial capabilities to nine. This evolution reflects a deeper understanding of how cancer cells not only acquire intrinsic functional advantages but also manipulate their environment to survive and thrive. The current model categorizes these parameters into three distinct dimensions: acquired functional capabilities, enabling phenotypic characteristics, and the role of the tumor microenvironment and systemic interactions.

The core of the framework consists of nine functional capabilities that cancer cells must acquire to sustain growth and progression. These include sustaining proliferative signaling (often through oncogene activation like KRAS or MYC), inactivating growth suppressors (such as TP53 or RB), and resisting programmed cell death. Furthermore, cancer cells must achieve replicative immortality, typically by activating telomerase or alternative lengthening of telomeres (ALT) to bypass the mitotic clock. These capabilities are not strictly linear; they are acquired through multistep tumorigenesis, where selective pressures favor cells that can circumvent the body's natural barriers to uncontrolled proliferation.

Modern cancer research emphasizes that tumors are not merely collections of mutant cells, but rather "outlaw organs." The framework now explicitly incorporates the tumor microenvironment (TME) and systemic interactions. This includes the recruitment of heterotypic cells—such as immune cells and fibroblasts—that are reprogrammed to support tumor growth. Enabling phenotypic characteristics, such as genomic instability and tumor-promoting inflammation, further facilitate the acquisition of these hallmarks. By viewing cancer through these interconnected dimensions, researchers can better identify therapeutic strategies, such as mechanism-guided co-targeting, to disrupt the dependencies that sustain these complex biological systems.