Six synthetic DNA control targets are included with every nCounter Gene Expression assay.  
Their concentrations range linearly (in *codeset*) from `128 fM` to `0.125 fM`, and they are referred to as **POS_A** to **POS_F**, respectively.  
These **Positive Controls** are typically used to measure the efficiency of the hybridization reaction, and their step-wise concentrations also make them useful in checking the linearity performance of the assay.

Since the known concentrations of the **Positive Controls** increase in a linear fashion, the resulting counts should, as well. 

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Note that because **POS_F** has a known concentration of `0.125 fM`, which is considered below the limit of detection of the system, it should be excluded from this calculation (although you will see that **POS_F** counts are significantly higher than the negative control counts in most cases).
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