A de novo (Latin for "from new") mutation is a genetic change that occurs for the first time in a person, and it wasn't inherited from either parent. The parents' DNA is normal; the mutation appeared spontaneously during the formation of the egg, sperm, or very early in embryonic development.
Why it matters for MCHS: Most MCHS cases are de novo, which means parents are typically not carriers and the risk of having another affected child is very low (though not zero, due to rare possibilities like germline mosaicism). This is important genetic counseling information for families.