CA 27.29 is a highly polymorphic glycoprotein that belongs to the mucin family and the product of the muc-1 gene. It is most effective using serial measurements to monitor the course of disease and the response to therapy due to the direct correlation of changing levels of CA 27.29 with clinical status. In patients with known metastases, a reduction in the levels of this marker indicates a good response to treatment while an increase in the levels indicate a resistance to therapy and progressive disease and justify further clinical evaluation and regular monitoring. It has recently been shown that an elevation of CA 27.29 levels above the upper limit of normal in patients with no clinical evidence of disease is an early indicator of recurrence. An elevated serum CA 27.29 level in patients in the remission of stage II or III breast cancer provided a positive predictive value of 83.3% for recurrent disease, with an average lead time of 5.3 months before recurrence was clinically established.