  Active frequency stabilization of a laser to an atomic or molecular resonance
underpins many modern-day AMO physics experiments. With a flat background and
high signal-to-noise ratio, modulation transfer spectroscopy (MTS) offers an
accurate and stable method for laser locking. Despite its benefits, however,
the four-wave mixing process that is inherent to the MTS technique entails that
the strongest modulation transfer signals are only observed for closed
transitions, excluding MTS from numerous applications. Here, we report for the
first time the observation of a magnetically tunable MTS error signal. Using a
simple two-magnet arrangement, we show that the error signal for the $^{87}$Rb
$F=2 \rightarrow F'=3$ cooling transition can be Zeeman-shifted over a range of
$>$15 GHz to any arbitrary point on the rubidium $\text{D}_2$ spectrum.
Modulation transfer signals for locking to the $^{87}$Rb $F=1 \rightarrow F'=2$
repumping transition as well as 1 GHz red-detuned to the cooling transition are
presented to demonstrate the versatility of this technique, which can readily
be extended to the locking of Raman and lattice lasers.
