Recent evidence suggests that many members of the human kallikrein gene family 
are differentially regulated in breast cancer and other endocrine-related 
malignancies. In this study, we utilised the serial analysis of gene expression 
(SAGE) and expressed sequence tag (EST) databases of the Cancer Genome Anatomy 
Project (CGAP) to perform in silico analyses of the expression pattern of the 15 
human kallikrein genes in normal and cancerous breast tissues and cell lines 
using different analytical tools such as Virtual Northern blotting, Digital 
Differential Display and X-profiler. Our results indicate that at least four 
kallikrein genes (KLK5, 6, 8, 10) are downregulated in breast cancer. Probing 
eight normal and 24 breast cancer SAGE libraries with gene-specific tags for 
each of the above kallikreins indicated moderate-to-high expression densities in 
normal breast (27-319 tags per million; tpm, in two to five out of eight 
libraries), compared to no or low expression (0 - 34 tpm in zero to two 
libraries out of 24) in breast cancer. These data were verified by screening the 
EST databases, where all mRNA clones isolated for these genes, except for one in 
each, were from normal breast libraries, with no clones detected from breast 
cancer tissues or cell lines (with the exception of KLK8). X-profiler comparison 
of two pools of normal and breast cancer libraries further verified the presence 
of significant downregulation of expression levels of 4 of the kallikreins genes 
(KLK5, 6, 10, 12). We experimentally verified the downregulation of these four 
kallikreins (KLK5, 6, 8, 10 and 12) by RT - PCR analysis.