  We consider the effects of a light quark mass in the soft-collinear effective
theory (SCET) and we apply them to B -> X_s gamma in the endpoint region. We
find that the reparameterization invariance can be extended by including the
collinear quark mass in the SCET Lagrangian. This symmetry constrains the
theory with the quark mass terms, and we present explicit results at one loop.
It also relates the Wilson coefficients of some mass operators to those of the
leading operators, which are useful in organizing the subleading effects due to
the quark mass in B -> X_s gamma. We present strange quark mass corrections to
B -> X_s gamma in the endpoint region as an application. The forward scattering
amplitude from the mass corrections is factorized, and it can be expressed as a
convolution of the m_s^2/p_X^2-suppressed jet function and the leading-order
shape function of the B meson. This contribution should be added to the
existing subleading contributions from the B meson shape functions to obtain
complete subleading corrections.
