To determine whether the retention of the M6P modification was specific to processed MPO in T47D and MCF7 cells or whether it is a general property of other enzymes stored in the lysosomes of these cells, we stained untransfected T47D and MCF7 cells for M6P content using the soluble fragment of the cation-independent mannose-6-phosphate receptor (sciMPR) and compared the staining to that of breast cancer cell lines in which no significant processing of MPO was observed. The gene discussed is MPO; the disease is breast cancer.