Is there a sentence fragment?
When scientists sequenced the DNA of domestic cats, they made a startling discovery. A remarkable similarity between the genomes of house cats and those of their feline relatives in the wild.

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 - no
There is a sentence fragment that does not express a complete thought.
When scientists sequenced the DNA of domestic cats, they made a startling discovery. A remarkable similarity between the genomes of house cats and those of their feline relatives in the wild.
Here is one way to fix the sentence fragment:
When scientists sequenced the DNA of domestic cats, they made a startling discovery: a remarkable similarity between the genomes of house cats and those of their feline relatives in the wild.