DNA CHIPS are silicon- or glass-based surfaces which are split up into regions onto which have been deposited (by, for example, lithographic or ink-jet-printing techniques) known sequences of DNA nucleotides (adenine, cytosine, guanine, and thymine) corresponding to those in genes of specific interest, such as the mutation-prone gene p53. The aim is that this DNA probe sequence should combine with a complementary sequence (an A nucleotide always binding with T, C always binding with G, etc.) of single strands of DNA from a sample injected onto the chip. These strands can be tagged with fluorescent chemicals so that they light up after being combined with the appropriate probe sequence positioned on the test bed. Thus, like a forensic comparison of fingerprints, the optical matchup of known and unknown genetic sequences can be performed in a methodical way and can rapidly detect such things as gene mutations in a sample of cells. And from this one can study genetically-based diseases. Researchers at Affymetrix, a company in California, have made chips with up to a million different chemical sequences. Scientists foresee the possibility of using several, or even single, DNA chips to detect mutations in perhaps all 100,000 human genes. For simpler organisms such as yeast, a DNA chip coded for the entire genome (6000 in this case) will be available soon. (Science News, March 8, 1997; San Jose Mercury News, November 26, 1996.)