$MIX variable from $PK

Dear NMusers, I am trying to get NONMEM to use a variable defined in $PK to be read in $MIX. Example partial code is shown below:$PKTVPROMIX=XX ; XX is not only a paramter to estimate, but is an equation $MIX PROMIX=TVPROMIX NSPOP=2 P(1) = PROMIX P(2) = 1 - PROMIX The error is as follows: AN ERROR WAS FOUND ON LINE 144 AT THE APPROXIMATE POSITION NOTED: PROMIX=TVPROMIX X THE CHARACTERS IN ERROR ARE: TVPROMIX 208 UNDEFINED VARIABLE. Whilst one would normally write everything of interest in $MIX, a problem arises when one wants to test covariates using PsN because the SCM only tests items defined in $PK/$PRED. Is there any way to have NONMEM read what is defined in $PK into $MIX? Much thanks, Mannie