Methods and diagnostic compositions for monitoring ER− breast cancer are disclosed. In some aspects, a method for detecting ER− breast cancer in a subject comprises obtaining a subject sample from a subject suffering from breast cancer; determining a level of NOHA in the subject sample; and comparing the level of NOHA obtained from the subject sample to the level of NOHA in a control sample, wherein a lower level of NOHA in the subject sample indicates presence of ER− breast cancer in the subject.