Complex sentence: Most individuals with Marfan syndrome have another affected family member--approximately 15--30% of all cases are due to "de novo" genetic mutations--such spontaneous mutations occur in about one in 20,000 births.
Split into simpler sentences: Most individuals with Marfan syndrome have another affected family member. Approximately 15--30% of all cases are due to "de novo" genetic mutations; such spontaneous mutations occur in about one in 20,000 births.