Fig. 3. Setting a discriminant function to maximally separate GPCRs from non-GPCRs in protein space. In a three-dimensional space, the function appears as a plane. The function was established using the training set of 750 known GPCRs and 1000 non-GPCRs. The function is used to classify novel proteins as either GPCRs or non-GPCRs, according to which side of the plane they map.