Expression differences between wheat 3' and 5' probesets. Comparison of expression profiles for wheat and barley allows an assessment of the reliability of the wheat 5' probesets. Homologous clusters of the type shown in Panel A suggest the simultaneous presence of both a 3' (W1) and 5' (W2) wheat probeset for the same gene that, at the same time, is homologous to a particular barley gene. The correlation of the expression profiles of these 3' and 5' probesets are shown in Panel B. On average, the profiles obtained from the 5' probesets show significantly lower correlation with that of their barley homologs than do the 3' probesets. This can also be seen in the distribution of the expression values themselves in Panel C. In general, expression values as measured by the 5' probesets tends to be much lower than those of their barley counterparts (solid line), while the signal from the corresponding 3' probesets shows a distribution roughly similar to that of the barley homologs.