Hierarchical clustering of differentially expressed probesets in parous and nulliparous women. Red represents expression values above the median across all samples, and green represents values below the median. In the two top-level clusters of samples, the right cluster is composed mainly of parous samples and the left cluster is composed mainly of nulliparous samples. ‘U’ represents the intensity of up-regulated probesets among parous samples whereas ‘D’ represents the intensity of down-regulated probesets. N represents Nulliparous and Y represents Parous. A chi-square test of independence on parity status and sub-tree membership resulted in a p-value of 0.001.