Products & Materials Products & Materials + See all authors and affiliations Science 19 Oct 1990:Vol. 250, Issue 4979, pp. 453DOI: 10.1126/science.250.4979.453 Article Info & Metrics eLetters PDF This is a PDF-only article. The first page of the PDF of this article appears below.
Science Vol 250, Issue 497919 October 1990 Table of Contents Print Table of Contents Back Matter (PDF) Ed Board (PDF) Front Matter (PDF)
