Histrograms for the number of probes per probe set. Both plots are histogram, the values on the y axis are relative frequencies. (top) Histogram of the number of probes on the U133A chip matching a particular RefSeq transcript. A clear peak can be seen for 11 probes, which corresponds to the number of probes in a probe set most commonly found in the original mapping. This set is called Alt1 in the last part of the section Results. (bottom) Histogram of number of RefSeq transcripts matching a single probe (log scale). Most of the probes can be seen matching only one RefSeq. In both plots, the probes associated to human 'ALU' repeats were filtered out.