A random string is a string in which a symbol is selected from the underlying alphabet based on a predetermined constant probability called the symbol frequency.

Random strings can be used to simulate genomes. If the genome's desired GC-content is $x$, then we select a 'G' or a 'C' with probability $\frac{x}{2}$ and select an 'A' or a 'T' with probability $\frac{1-x}{2}$.