BRCA1 and breast cancer: As one example of the latter process, a researcher may ask whether a dataset supports her hypothesis that breast cancer relapse will be associated with high levels of PSRC1 expression or a combination of low levels of PSCR1 and high levels of BRCA1. This hypothesis can be represented as a decision tree rooted in a feature for PSRC1 gene expression with a leaf node predicting relapse for high PSCR1 and a secondary split under low PSCR1 on BRCA1 expression (Fig. 1).