Placental-specific protein 1 (PLAC1) is an X-linked trophoblast gene that is re-expressed in several malignancies, including breast cancer, and is therefore a potential biomarker to follow disease onset and progression. Sera from 117 preoperative/pretreatment breast cancer patients and 51 control subjects, including those with fibrocystic disease, were analyzed for the presence of PLAC1 protein as well as its expression by IHC in tumor biopsies in a subset of subjects. Serum PLAC1 levels exceeded the mean plus one standard deviation (mean+SD) of the level in control subjects in 67% of subjects with ductal carcinoma in situ (DCIS), 67% with HER2+ tumors, 73% with triple-negative cancer and 73% with ER+/PR+ tumors. Greater sensitivity was achieved using the mean+2 SD of control PLAC1 serum values, where the false positive rate was 3% and was exceeded by 38%, 40%, 60% and 43% of subjects with DCIS, HER2+, TNBC and ER+/PR+/HER2- tumors. PLAC1 was detected in 97% of tumor biopsies, but did not correlate quantitatively with serum levels. There was no significant correlation of serum PLAC1 levels with race, age at diagnosis, body mass index (BMI) or the presence of metastatic disease. It remains to be determined whether PLAC1 serum levels can serve as a diagnostic biomarker for the presence or recurrence of disease post-surgery and/or therapy.