Overexpression of cathepsin D in human breast cancers is associated with a higher risk of relapse and metastasis. Also, pro-enzyme routing is altered in several tumoral mammary cell lines, leading to its hypersecretion. MCF7 cells compared to normal kidney carry a C-->T transition at position 224 in the cathepsin D gene which converts Ala to valine in its pro-fragment. Using polymerase chain reaction-single strand conformational polymorphism analysis (PCR-SSCP), the variant T allele frequency was found to be 23-30%, and equally distributed in cancer and normal cells. Six to nine per cent of genotypes were homozygous T/T, 34-41% were heterozygous T/C and 50-59% were homozygous C/C. Moreover, genotypes were identical in 19 out of 20 matched sets of tumoral mammary cells and normal white blood cells from the same patients. Loss of heterozygosity was noted in 1 case. C/T224 transition is thus not due to a somatic event. However, this missense polymorphism might modify procathepsin D secretion and/or maturation in breast cancer cells.