What is the difference between a PSR and an Exon?
PSR is the smallest unit on the exon array for expression profiling and each PSR is represented by an individual probe set. In some cases, each PSR is also an exon; in other cases, due to variation in overlapping exon structures, the PSR can be a subset of the true biological exon. As a result, alternatively spliced exons from the same gene may overlap (i.e., alternative donor or acceptor site); however, PSRs have the property that they do not overlap each other in the genome space, except if annotations change with a newer version of the genome assemblies. In cases where multiple annotations infer different exon structures, that one exon cluster (a group of overlapping exons) will be divided into multiple PSRs. Therefore, in the final design, there are approximately 1,000,000 exon clusters represented by approximately 1,400,000 PSRs.
