Editor:

    Robert L. Perlman
    Richard L. Landau

    Book Editors:

    Morton Arnsdorf

    Associate Editors:

    Clifford W. Gurney
    Ronald Singer

    Managing Editor:

    Solveig C. Robinson

    Advisory Board:

    Charlotte Auerbach Edinburgh
    Baruch Blumberg Philadelphia
    C. di Duve Brussels
    Roger Guillemin San Diego
    Elwood Jensen Hamburg
    Robert L. Kroc Santa Barbara
    Rosalyn S. Yalow New York

    Editorial Board:

    Daniel A. Albert Chicago
    Nobuyuki Amino Osaka
    William S. Beck Boston
    Paul B. Beeson Seattle
    Douglas Black London
    K. M. Brinkhous Chapel Hill
    Donald Chambers Chicago
    Paul W. Ewald Amherst
    Nina Fedoroff University Park
    Renee Fox Philadelphia
    James M. Gustafson Atlanta
    Walter B. Gratzer London
    Robert J. Joynt Rochester
    Maclyn McCarty New York
    Victor A. McKusick Baltimore
    James V. Neel Ann Arbor
    C. J. V. Nossal Melbourne
    Sherwin Nuland New Haven
    Alan L. Pearlman St. Louis
    Gert H. Brieger Baltimore
    T. N. Pullman Chicago
    Alan N. Schechter Bethesda
    M. G. P. Stoker London
    Editorial correspondence and manuscripts should be sent to Dr. Robert Perlman, Editorial Office, Perspectives in Biology and Medicine, Culver Hall 404, 1025 East 57th St., Chicago, IL 60637. See "Notes to Contributors" for more information about preparation and submission of manuscripts. Fax: 773-702-9234. Email: r-perlman@uchicago.edu.

    Editorial Sponsor: THE DIVISION OF THE BIOLOGICAL SCIENCES, The University of Chicago.

    Perspectives in Biology and Medicine (ISSN 0031-5982) is published quarterly in Autumn, Winter, Spring, and Summer by THE JOHNS HOPKINS UNIVERSITY PRESS.

    Postmaster: Send address changes to Perspectives in Biology and Medicine, The Johns Hopkins University Press, Journals Division, 2715 North Charles Street, Baltimore, MD 21218-4363.

    Subscription rates, U.S.A.: individuals, 1 year $39.00; students, 1 year $34.00 (photocopy of valid student ID must accompany subscription); institutions, 1 year $85.00. Other countries add $4.00 for each yearŐs subscription to cover postage. Single copy rates: individuals $12.00, institutions $25.50. (Canadian residents, please add 7% GST to all rates listed above.) Volumes in microfilm available from Bell & Howell Information & Learning, PO Box 1346, Ann Arbor, MI 48106 (800-521-0600 or 734-761-4700). Subscription correspondence should be sent to The Johns Hopkins University Press, Journals Division, PO Box 19966, Baltimore, MD 21211-0966. Subscriptions are payable in advance. Please make all remittances payable to THE JOHNS HOPKINS UNIVERSITY PRESS.

    All rights reserved. No portion of this journal may be reproduced by any process or technique without the formal consent of The Johns Hopkins University Press. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by The Johns Hopkins University Press for libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service, provided that the base fee of $8.00 per article is paid directly to CCC, 222 Rosewood Drive, Danvers, MA, 01923. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. 0031-5982/00 $8.00

    Advertising space in Perspectives is available, as is rental of its subscriber list. For information and rates, please con-tact the advertising sales staff, The Johns Hopkins University Press, Journals Division, 2715 North Charles Street, Baltimore, MD 21218-4363 (410-516-6988).

    For indexing/abstracting information, see http://muse.jhu.edu/journals/pbm.

    Periodicals postage paid at Baltimore, MD, and at additional mailing office.

    © 2000 by The Johns Hopkins University Press. All rights reserved.

    This publication is printed on acid-free paper.