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. This evidence concerns the gene MPO and breast carcinoma.