The probes on the HG-U133 Set are designed to detect the anti-sense strand of the gene of interest, and these probes are annotated with the "_at" extension. The primary goal in probe set selection is to select a probe set unique to a single transcript or common among transcripts from the same gene.

Often, it is not possible to select a probe set that is unique to a single transcript because multiple transcript variants share common sequence. Such probe sets are annotated with the "_s_at" extension. Occasionally, it is not possible to select a unique or shared probe set. Such probe sets are annotated with the "_x_at" extension. For more information, please review the _s_at and _x_at_ probe set descriptions.

Please note: There are no "_f_at", "_g_at", or "_r_at" probe sets in the HG-U133 design, as opposed to the HG-U95 Array design. This is because the nonunique probe set types were simplified and adjusted to account for improvements due to improved probe selection rules.