A microarray is a chip covered with thousands of tiny spots of DNA or protein. Each spot corresponds to a specific gene or protein. When a sample is applied, the molecules in the sample bind to their complementary spots. The amount of binding is measured, giving a snapshot of gene expression or protein levels across thousands of targets at once. The technology was a revolution when it appeared in the late 1990s.

The most common type is the DNA microarray. It can measure the expression of every gene in a genome in a single experiment. It can also detect genetic variations, like single nucleotide polymorphisms, and copy number changes. Protein microarrays measure protein levels and interactions. Tissue microarrays allow pathologists to analyze many tissue samples on one slide.

Microarrays have been largely replaced by RNA sequencing for gene expression studies, because sequencing is more sensitive and can detect novel transcripts. But microarrays remain useful for high-throughput genotyping, clinical diagnostics, and applications where cost matters. They are also used in research to validate sequencing results.

The technology is mature and widely available. It requires careful experimental design and data analysis. A microarray experiment can generate more data than a single lab can interpret, which is why bioinformatics is essential.