Nucleomorphs are small, reduced eukaryotic nuclei found in certain plastids. So far, only two groups of organisms are known to contain a nucleomorph: the cryptomonads and the chlorarachniophytes. The nucleomorphs support the endosymbiotic hypothesis, and are an evidence that the plastids of these organisms are so-called complex plastids. Studies of the genomic organization and of the molecular phylogeny have shown that the nucleomorph of the cryptomonads formerly was the nucleus of a red alga, whereas the nucleomorph of the chlorarchniophytes formerly was the nucleus of a green alga. In both groups of organisms the plastids originate from engulfed photoautotrophic eukaryotes. After the red or green alga was engulfed by its host cell, it was reduced. Nucleomorphs retained only three chromosomes and many genes were transferred to the nucleus of the host cell. The unique combination of host cell and complex plastid results in cells with four genomes: two prokaryotic genomes (mitochondrion and plastid of the red or green alga) and two eukaryotic genomes (nucleus of host cell and nucleomorph).