[R] extension to nlme self start SSmicmen?

[David Airey]

Dear list,

Has anyone created a version of SSmicmen that allows testing for group  
differences? The basic Michaelis-Menten equation is:

(Bmax * X) / (Kd + X).

The nlme package allows modeling of random effects for Bmax and Kd as  
needed, but I curious how I can build in group differences? I have  
receptor binding data for strains of mice, and following Pinheiro and  
Bates' lead in their book, a random effort for animal differences in  
Bmax seems reasonable, within strain. However, I'd like a self-start  
function, or even just the equation above, modified to allow testing  
for strain differences in Bmax and or Kd.

Thank you.

-Dave