Mutually modulated cross-gain modulation (MMXGM) is a new effect which we have demonstrated in optical fiber, mediated by stimulated Brillouin scattering (SBS). In this effect, a modulated pump and modulated signal beam interact, resulting in a change in the modulation phase of the output signal.  This modulation phase is sensitively dependent upon various parameters, such as the optical frequency of each of the beams.  We report on the appearance of MMXGM mediated by two widely different nonlinear mechansims: SBS in an optical fiber and cross-gain mixing in a semiconductor optical amplifier, and show experimental results of its' highly sensitive dependence on optical frequency.  This is a new means of transforming optical frequency changes to modulation-phase changes.