For any particular array set, there are probe sets for only a subset of all possible sub-clusters. Moreover, some sequences are excluded because they are of low quality or are problematic (for example, too short or too repetitive), while other sequences have been added to the public databases since the array was designed. If you cannot find your sequence identifier, try running BLAST to search for a similar sequence or using the Probe Match tool. Please note: You have to search all the sub-clusters from a UniGene cluster before concluding that your sequence is missing in the sub-cluster databank.