Figure 1. Possible epidemiological scenarios accounting for the observed increased incidence of early onset colorectal cancer. (A) An environmental exposure occurs in the total population, but an increase in colorectal cancer is not seen in patients that are screened, with an apparent increase in patients solely because that group is not screened. This scenario might imply genetic and epigenetic factors and pathways are like those observed in later onset colorectal cancer. (B) An environmental exposure occurs, but the exposure selectively affects younger populations over older populations (through accelerated pathogenesis), providing a true increase in colorectal cancer in that population without affecting older populations. This pathway might imply genetic and epigenetic factors and pathways that are distinct from those observed in later onset colorectal cancer. This scenario also implies that tumor initiation and possibly tumor progression are accelerated over the typical progression timeframes observed for later onset colorectal cancer.