Completion of the human genome sequence has made it possible to study the expression of the entire human gene complement (>30,000 estimated genes). Aiding in this remarkable feat, DNA microarrays have become the main technological workhorse for gene expression studies. To date, detection platforms for most microarrays have relied on short (25 base) oligonucleotides synthesized in situ, or longer, highly variable length DNAs from PCR amplification of cDNA libraries. A third choice, long (50-80 base) oligonucleotide arrays, is now available and might eventually eliminate the use of cDNA arrays. The technology has advanced to such a point that researchers now demand microarrays that are cost-effective and have flexibility and quality assurance. Short- and long-oligonucleotide technologies offer such advantages, and could possibly become the major competing platform in the near future.