Hi Sarah, for a homogeneous population of binding sites the binding or association constant (K_a) = (k_on / k_off) - the on and off rates. The dissociation constant (K_d) is the inverse of this. So measuring K_a and the binding rate allows you to calculate k_off. Bear in mind however that MIPs are not generally homogeneous binders. I hope this helps. Can anyone else help Sarah? Mike Whitcombe (SMI President)