When multiple human genomes are compared, there are variations observed in the sequences. Variations due to the substitution of one nucleotide are called single nucleotide polymorphisms, or SNPs. Variations due to the insertion or deletion of a sequence of nucleotides less than one kilobase in length are called an indel. If an insertion or deletion of nucleotides is copied a variable number of times in the same genome, it's called a copy number variation or CNV. Often, CNVs involve large stretches of DNA greater than one kilobase in length. A haplotype is a set of genes inherited from a single parent. A genome can be divided into haplotype blocks that contain clusters of polymorphisms. Since few crossovers occur between homologous chromosomes during each meiosis, haplotype blocks have been inherited in a linked group across generations.