In the previous Tetramer Tips blog, I suggested ways for you to be a negative Control Freak. Now it’s time to think positive! A positive control for a tetramer is a sample that contains cells expressing the specific T cell receptor of interest, i.e. has the exact specificity of the tetramer. Okay, brace yourselves; I’m going to be frank here. Having a positive control for tetramer experiments is often a quest for the Holy Grail. In many (most, in fact) cases, you will not have access to a positive control, unless one of your experimental samples happens to show a positive result. In an experiment where no positive events are seen and no positive control was used, you cannot necessarily conclude that the donor/patient/mouse is negative for that T cell specificity, because, heck, maybe you got distracted while pipetting and forgot to add the tetramer to your staining cocktail! A tetramer experiment with no positive control and no positives in the experimentals is, therefore, uninterpretable. This is the hard truth we must face.