Distribution of OR transcripts and probesets in the human genome. A total of 867 transcripts and 206 probesets expressed in non-olfactory tissues, are shown in their approximate genomic location. OR clusters (as defined in HORDE ) are indicated on the chromosomes in dark grey rectangles with their width scaled to the number of ORs in the cluster. Nearby clusters may be merged. Black bars on the right specify the number of transcripts (ESTs or mRNAs) per cluster. Blue chains on the left represent the number of microarray expressed probesets in the cluster. Since distribution of clusters over chromosomes is not uniform, bars/chains are sometimes very close.