(CNN) You've probably heard of identical and fraternal twins, but a report published Thursday says there's a third kind -- sesquizygous twins or "semi-identical."

Identical -- or monozygotic -- twins pop up from a single fertilized egg that eventually splits in two and forms two identical boys or two girls. They share 100% of their DNA.

Fraternal -- or dizygotic -- twins form from two eggs that have been fertilized by two of the father's sperm, producing two genetically unique siblings. They share 50% of their DNA.

But "semi-identical" twins are so rare, experts say they have only identified two cases -- ever.

Right along that DNA-sharing spectrum, "semi-identical" twins share anywhere from 50% to 100% of their genomes, researchers say.

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