A schematic illustration of COXEN algorithm by using an artificial five-probe example. Probes 1 and 3 in cell set show essentially the same patterns as probes 1 and 3 in cell set 2. These two probes have the same co-expression correlation with other probes in these two cell sets. On the contrary, probes 2, 4, and 5 show different patterns of co-expression correlation in these two cell sets. Therefore, Probes 1 and 3 (but not 2, 4, and 5) will be selected by the COXEN algorithm.