Semi-identical twins are rare, and doctors say they've identified the second case ever

(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.
      And they're extremely, extremely rare. The only other reported case of sesquizygotic twins was reported in the United States in 2007. The recently identified twins from South East Queensland are now 4 years old and healthy.
      Details of this second case were published this week in the New England Journal of Medicine. Researche