A gene is a unit of inherited biological information that influences various functions in an organism. A gene is said to be expressed if it is active in the sense of producing material that can potentially start, stop, or modify an organism’s chemical, physiological, or behavioral functions. Microarray technology enables the simultaneous measurement of the expression level of each of thousands of genes from a biological sample drawn from a tissue such as the bone tissue of an animal or the stem tissue of a plant. Statistical analysis of microarray data and other types of genomics data is based on analogies between real measurements and random numbers. These analogies are called sampling models or error models. In a case-control study of a human population, a genetic marker records the allele inherited from each parent at a specific DNA site called a variant or single-nucleotide polymorphism.