Cancer antigen 125 looks for which cancer?

Ovarian, but... An elevated ca125 can be due to many things. These include gallbladder disease, ovarian cysts, inflammatory bowel disease, endometriosis, or anything that causes inflammation inside the abdomen. And it can be normal with ovarian cancer. This means it's not a good test to determine if you have ovarian cancer, or to determine that you don't.
Ovarian cancer. Although not reliable as a screening treat, ca125 can be helpful in monitoring a patient with known ovarian cancer. There are too many things other than ovarian cancer that affect ca125 to make it useful in general population screening. It is helpful in screening high risk patients due to genetic factors.