 PGH differs from common PGD methods such as fluorescence "in situ" hybridization (FISH) and polymerase chain reaction (PCR) for two primary reasons.First, rather than focusing on the genetic makeup of an embryo PGH compares the genome of affected and unaffected members of previous generations.This examination of generational variation then allows for a haplotype of genetic markers statistically associated with the target disease to be identified, rather than searching merely for a mutation.