Haploid cells are cells that contain a single set of chromosomes. In humans, only the egg and sperm cells are haploid, while most cells in the human body are diploid, meaning they have two copies of each chromosome. Haploid gametes are produced during meiosis, which is a type of cell division that reduces the number of chromosomes in a parent diploid cell by half. The number of chromosomes in a single set is represented as n, which is also called the haploid number. In humans, n = 23. The union of gametes during fertilization creates a diploid cell called a zygote with two sets of chromosomes. Haploid cells are useful tools to study gene function as they contain a single copy of the genome and are thus unable to mask the effect.