S-methyl DM1 is the primary cellular or liver metabolite of antibody-maytansinoid conjugates prepared with thiol-containing maytansinoids DM1[1]. The half-maximal concentration for inhibition of microtubule assembly for for S-methyl DM1 is 4 μM. At 100 nM S-methyl-DM1 (84%) suppresses dynamic instability more strongly than Maytansine (45%). Tritiated S-methyl-DM1 bound to 37 high-affinity sites per microtubule (Kd of 0.1 μM)[1]. Minimal inhibition occurred at 200 pM S-methyl DM1, and inhibition is maximal at 3 nM. S-methyl DM1 (IC50 of 330 pM) is slightly more potent than Maytansine (IC50 of 710 pM)[2]. S-methyl DM1 induces maxima of 80% accumulation of cells in G2/M as compared with only 30% in controls in MCF7 cells[2].