The human genome contains approximately 21,000 genes. At any given moment, each of our cells has some combination of these genes turned on, and others are turned off. How do scientists figure out which are on and which are off?

Scientists can answer this question for any cell sample or tissue by gene expression profiling, using a technique called microarray (pronounced MY-crow-ah-ray) analysis.

Microarray analysis involves breaking open a cell, isolating its genetic contents, identifying all the genes that are turned on in that particular cell, and generating a list of those genes.