While most chloroplasts originate from that first set of endosymbiotic events, Paulinella chromatophora is an exception that acquired a photosynthetic cyanobacterial endosymbiont more recently. It is not clear whether that symbiont is closely related to the ancestral chloroplast of other eukaryotes. Being in the early stages of endosymbiosis, Paulinella chromatophora can offer some insights into how chloroplasts evolved. Paulinella cells contain one or two sausage shaped blue-green photosynthesizing structures called chromatophores, descended from the cyanobacterium Synechococcus. Chromatophores cannot survive outside their host. Chromatophore DNA is about a million base pairs long, containing around 850 protein encoding genes—far less than the three million base pair Synechococcus genome, but much larger than the approximately 150,000 base pair genome of the more assimilated chloroplast. Chromatophores have transferred much less of their DNA to the nucleus of their host. About 0.3–0.8% of the nuclear DNA in Paulinella is from the chromatophore, compared with 11–14% from the chloroplast in plants.