Complex sentence: For reasons not yet understood, schwannomas are mostly benign and less than 1% become malignant, degenerating into a form of cancer known as malignant peripheral nerve sheath tumour, malignant schwannoma, or neurofibrosarcoma.
Split into simpler sentences: For reasons not yet understood, schwannomas are mostly benign and less than 1% become malignant (degenerate into cancer). This form of cancer is known as malignant peripheral nerve sheath tumour or malignant Schwannoma or neurofibrosarcoma.