Expression analysis of 107 presynaptic genes across mouse tissues and developmental stages. Expression profile of 107 genes corresponding to 161 oligonucleotide gene probes were clustered into 3 distinct expression groups by the hierarchical k-means clustering method. (a) Cluster 1: transcripts with high expression in brain tissues and low levels of expression in other tissues. (b) Cluster 2: transcripts with moderate expression in brain tissues and low levels of expression in other tissues. (c) Cluster 3: transcripts with widespread but low levels of expression in most tested tissues.