DNA microarrays work by exploiting the specificity of DNA base pairing. The initial rules for hybridization were discovered by Erwin Chargaff and dictate that each guanine noncovalently pairs with a cytosine and each adenine is paired with a thymine . The affinity and stability of the hybridized, double stranded DNA is therefore directly related to sequence complementarity. In this figure the labeled "target" molecules, representing the mRNA transcripts, compete for binding to their complementary "probe" molecules immobilized on the array. Once equilibrium is achieved, the arrays are washed and scanned to measure the transcript abundance.