The custom CDF was built by uniquely mapping 232,697 probe sequences to 21,389 Arabidopsis (TAIR8) gene-based probe sets in the following manner: (i) probes that have perfect sequence identity with a single target gene were selected, (ii) probes mapping to reverse complements of genes were annotated separately as antisense probes (not used in the above counts), and finally, (iii) probes were grouped into probe sets, each corresponding to a single gene, and probe sets having at least 3 unique sequences were retained (>99% probe sets have >=5 probes). Note that these stringent criteria used to construct the CDF make it possible to reliably measure expression values of members of multigene families (free from cross-hybridization between paralogs showing high sequence similarity).