Four GxE interaction models provided by the Knowledge-Aided Parameterization System (KAPS): (a) a genetic model, (b) an environmental model, (c) a gene-environment interaction model and (d) an additive model. Each curve represents the relationship between disease risk (y-axis) and an environmental factor (x-axis) for individuals with a particular genotype (AA, Aa or aa) at a disease-predisposing locus (DPL). The environmental model has only one curve because the relationship between environmental exposure and disease risk is identical for all genotypes. Adapted from Amato et al. .