 The chiral induced spin selectivity, CISS, effect in which chiral molecules act as spin filters that preferentially transmit electrons with spin polarized parallel or anti-parallel to their direction of motion can be included in conventional models of radical pair spin dynamics. CISS can increase the sensitivity of radical pairs to the direction of a weak external magnetic field, change the dependence of the magnetic field effect on the reaction rate constants, and destroy the field inversion symmetry characteristic of the RPM. The associated lack of field inversion symmetry has not hitherto been observed in behavioral experiments, and it may no longer be safe to assume that the observation of a polar magnetic compass response in an animal can be used as evidence against a radical pair sensory mechanism. This article was authored by Giotto Luo and P. J. Ho.