What does 'differentially express' mean in this sentence?

Question

Students will work with tumour cell lines that differentially express HER2 and that are used as quality controls in diagnostic laboratories.

(HER2 is overexpressed in some breast cancers)

Answer 1

Different cell lines will express HER2 in different amounts.  You just mentioned that some breast cancers overexpress HER2.  The different cell lines will express the HER2 differently.  In fact, it might be a good way to tell the cell lines apart from one another.  For example, one line expresses a lot of HER2, and another line not as much.  You can determine in the lab how the HER2 is expressed by staining for it, or possibly by immunoassay.

IF this is being used as a "quality control" it is being used as a standard to test where a normal "ought" to be in comparison to the different cell lines.  This is like having the urine glucose test strip, where different amounts of sugar make a different shade of color on the urine strip.  The "normal" level is compared with the colors on the strip to tell whether it is higher or lower than expected.  Think of the differentially expressed cell lines as colors on a test strip, you will compare samples with these to see if they express more or less HER2 than the controls.