  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 new material parameter. We extract its value from a recent experiment.
We also present a very simple microscopic theory predicting a value comparable
to the one observed. Finally we sketch the effect of constrained twist on
entropic elasticity of DNA arising from the connection between Link, Twist, and
Writhe.
