slides were immersed in diaminobenzidine (DAB) (Sigma). The staining process was terminated by removal of DAB, after which the slides were washed in H2O or PBS. The sections were subsequently stained with Mayer’s haematoxylin (Bio-Optica) for 2 5 s, rinsed in cold-tap water for 15 min, and dehydrated by immersion in increasing concentrations of ethanol (70%, 95%, 100%) and finally in xylene and mounted with Pertex. Results In this work, the protein expression of phosphohis- tidine phosphatase, PHPT1, was investigated by immunohistochemistry, using an affinity-purified polyclonal PHPT1 antibody on sections of paraffin- embedded mouse tissues. Samples were taken from both E14.5 embryos and adult mice of the C57BL/ 6J or B6CBAF1 strains. Concomitantly, the anti- bodies used in the present study were included in the HPR project, where expression of proteins in normal and cancerous human tissues and cell lines is determined. Some of the mouse results are illustrated in Figures 1 and 2, and the human results can be found at http://www.proteinatlas.org/index.php. The mouse tissue sections were examined, and the stain- ing intensity was ranked from ( ), for no staining, to ( ) for a strong signal. In the case of mouse tissues, the results were confirmed in at least two additional series using tissues from different mice. The PHPT1 protein showed a tissue-specific expres- sion pattern in mouse and human tissues, and the results are summarized in Table I. Specificity of the PHPT1 antibody To evaluate the specificity of the PHPT1 antibody, a peptide absorption test was performed in which PHPT1 peptide 101-115 was added to the PHPT1 antibody before incubation with the sections. The absorption test dramatically reduced the immunor- eactive signals. An example is seen in Figure 2D, when compared with Figure 2C. Preimmune sera were, in several experiments, used in parallel, and no staining was obtained. An example is seen in Figure 1B. These results confirmed that the binding of the antibody to the PHPT1 peptide was specific. PHPT1 expression in mouse embryo In the mouse E14.5 sections, strong PHPT1 im- munostaining was seen in the epithelium of choroid plexus of brain ventricles and at the surface epithe- lium of the gastrointestinal tract (Figure 1 and Table Immunohistochemical localization of PHPT1 67 I). The embryonic heart muscle (Figure 1E) and skeletal muscle also expressed PHPT1, while the control sections incubated with preimmune serum did not demonstrate any staining (Figure 1B). PHPT1 expression in adult tissues Brain. In the adult mouse brain, PHPT1 was specifically expressed in the epithelium layer of the choroid plexus, while the mesenchymal tissue of choroid plexus did not exhibit any expression (Figure 1C). In cerebellum, the Purkinje cells clearly showed PHPT1 signals also in the nuclei (Figure 1D). There were also small groups of cells scattered in the mid-brain and hippocampus regions that were stained for PHPT1 (data not shown). The identity of these latter cell types is not yet clear. In the cerebral cortex of adult mouse, no cells that demonstrated clear PHPT1 expression were seen. Gastrointestinal tract including liver. Weak to moderate level of PHPT1 expression was seen in the gastro- intestinal surface epithelium of the digestive tract, both in adult mouse and in human (Table I). PHPT1 was clearly expressed in the epithelium of mouse ducts of internal organs. A moderate PHPT1 expression was seen in the bile duct epithelium of the adult mouse liver, whereas a weak signal was seen in the hepatocytes (data not shown). The Kupffer cells and endothelial cells of liver did not stain for PHPT1. In adult mouse pancreas, the epithelium of interlobular ducts specifically and strongly ex- pressed PHPT1, while the mouse acinar cells showed no or only a weak signal (Figure 2E). The human acinar cells, on the other hand, displayed a signal at a moderate level. The islet endocrine cells weakly expressed PHPT1 in the adult mouse and moderately so in the human tissue (Table I). Lung. In the adult mouse lung, the capillary en- dothelial cells of alveolar septa stained negatively for PHPT1. Alveolar macrophages in the alveoli ex- pressed PHPT1 with a weak to moderate signal. The epithelium of bronchioles and bronchi also ex- pressed PHPT1 (Figure 2F). Testis. No staining was observed in the spermatogo- nia located on the basement membrane surrounding the seminiferous tubules, whereas the labelling of spermatocytes and spermatids (that is, later stages of spermatogenesis) was moderate (Table I, Figure 2C). Interestingly, in human testis, spermatogonia moderately expressed PHPT1 with a somewhat lower expression in the spermatocytes and a weak expression in spermatids. Further, human interstitial cells of Leydig moderately expressed PHPT1, which was in contrast to those in adult mouse testis (see interstitial cells in Table I). In mouse, the epithelium