  The symmetries of the DNA double helix require a new term in its linear
response to stress: the coupling between twist and stretch. Recent experiments
with torsionally-constrained single molecules give the first direct measurement
of this important material parameter. We extract its value from a recent
experiment of Strick et al. [Science 271 (1996) 1835] and find rough agreement
with an independent experimental estimate recently given by Marko. We also
present a very simple microscopic theory predicting a value comparable to the
one observed.
