I agree with David, you may want to consider having an additional panel on hand. Using your screening cells in your rule outs or as one of your three positives/negatives may help but again, if your dealing with something other than a K or E you may have trouble. How are you doing your ruling out? We prefer to rule out cells with homozygous expression. If we have to use heterozygous we require two negatives, (when using gel or peg). When doing rule outs and using LISS as the testing media, we do not use cells with heterozygous expression.