What is the type of mutation that causes bloom's syndrome?

This mutation deletes six DNA building blocks (nucleotides) and replaces them with seven others at position 2281. The blmAsh mutation results in the production of an abnormally short, nonfunctional version of the BLM protein. This called a missense mutation.
The mutation is recessive and both parents must carry it to produce a child with the disorder. It is found in populations where there are close relatives that marry.
Other BLM gene mutations change single protein building blocks (amino acids) in the protein sequence or create a premature stop signal in the instructions for making the protein.