[R] missing values in mixed model

[Irene Mantzouni]

Dear all,
 
I have a mixed model of the form:
 
y[it]=a+b*z[i]+ai+(c+c[i])*x[it] ("i" refers to group and "t" to observation in group). 
zi are group-specific variables available only for a number of groups
and "ai", "ci" denote the random effects on the intercept and slope respectively.
 
Is there some way to drop b*zi for the missing value cases or some other way to deal with them? 
 
thank you!
 
Irene