Complex sentence: Doctors can measure the levels of the CA-125 protein in a woman's blood; high levels could be a sign of ovarian cancer, but this is not always the case, and not all women with ovarian cancer have high CA-125 levels.
Split into simpler sentences: Doctors can measure the levels of the CA-125 protein in a woman's blood--high levels could be a sign of ovarian cancer, but this is not always the case. And not all women with ovarian cancer have high CA-125 levels.