Complex sentence: This clone proliferates and may slowly evolves into more aggressive sub-clones that cause smouldering multiple myeloma and with further and more rapid evolve cause the overtly malignant stage of multiple myeloma and subsequent extremely malignant stage of plasma cell leukemia.
Split into simpler sentences: This clone proliferates and may slowly evolve into more aggressive sub-clones that cause smouldering multiple myeloma. Further and more rapid evolution causes the overtly malignant stage of multiple myeloma and can subsequently lead to the extremely malignant stage of secondary plasma cell leukemia.