carrying over values from one variable if missing in another

I want to carry over (or copy) values from one variable to another, but only if they are missing values in the second variable.

Could you tell me what syntax I should use?

Is it
if var2=99 var2=var1 ??

Thanks a lot
That would work. For a more detailed exposition on missing values look up missing values under "Universals" in the command syntax reference manual which can be found via the SPSS help facility.