Mapping clusters of genes of GO functions transcriptionally susceptible to c-secretase activity levels resulted in a GO hierarchydependent tree that will provide further orientation for c-secretase research. Functional clustering of 2658 differentially expressed sequences (1981 genes, Supplemental Material, Dataset S3) was performed using the FatiGo tool [23] . Comparing the representation of functional groups of genes throughout the entire mouse genome with their representation within the group of differentially transcribed genes allowed us to see whether clusters of genes of a specific functional group were enriched in the differentially expressed set. Clusters of over-and underrepresented genes were detected (Fig. 2) . The gene functions ''transcription regulator activity'', ''kinase regulator activity'', ''catalytic activity'' and ''binding '' were found to be overrepresented among the 2658 sequences (1981 genes) that were differentially transcribed. The cluster of ''molecular transducer activity'', through its subclusters in the GO hierarchy: ''receptor activity'' GO0004872, ''transmembrane receptor activity'' GO 0004888 and ''neurotransmitter receptor activity'' GO0030594, as well as the cluster of ''transporter activity'', via its subcluster of ''ion transporter activity'' GO0015075, were underrepresented (Fig. 2 , blue boxes). This is significant since neurotransmitter activity and transmembrane receptors are well within the focus of current AD research [24] .