direction of the expression units in multi-gene vectors - (Jul/02/2014 )

Hi

I am going to construct an mammalian expression plasmid with 3 expression unit 2 are coding heterologous proteins and 1 is for the mammalian antibiotic resistance gene. i am trying to use the backbone of l vectors like pcDNA3.1. the vector already has the antibiotic resistance unit and I can clone one of the genes in the MSC of vector so I will only need to add one expression unit (CMV -GOI-Poly A) to the vector but  I was wondering if the direction and location of this expression unit could be  important for the expression efficiency of the transgenes. 

Bests

As you are including a separate promoter, I don't think the direction will matter too much.