68 X.-Q. Zhang et al. Figure 1. Immunohistochemical staining for PHPT1. A: An E14.5 mouse embryo sagittal section shows PHPT1 signal in the epithelium of the choroid plexus in the fourth ventricle of brain (arrow). B: Adjacent section of A, using preimmune serum as a negative control. C: The same expression pattern of PHPT1 was found in the epithelium layer of the choroid plexus in an adult mouse brain (arrow). Arrowheads point at the ependymal cells in the ventricle. D: Purkinje cell (arrow) in the cerebellum expressing PHPT1. E: PHPT1 expression in the E14.5 embryonic heart muscle. F: PHPT1 expression (arrow) in the epithelium layer of the developing gut of an E14.5 sagittal section. Ampliﬁcations were 100 for A, B, and E, 400 for C and D, and 25 for F. of the duct of the epididymis showed moderate PHPT1 expression (Table I). Kidney. PHPT1 expression in adult mouse kidney was mainly restricted to the cortex. PHPT1 was not expressed in any parts of the glomeruli (Figure 2A). It was very weakly expressed in the epithelium of the proximal tubuli, but was clearly and strongly ex- pressed in the epithelium of Henle’s loop and distal tubule (Figure 2A and B). However, the strong expression of PHPT1 was dramatically decreased, or even lacking, in the collecting tubuli in the medulla. No or weak expression of PHPT1 was seen in the renal papilla and calyx. Muscle. PHPT1 was also expressed in striated muscle of adult mouse, including both skeletal and heart muscle. The smooth muscle of the gastrointestinal tract and blood vessels also ex- pressed PHPT1 (Table I). Overview and intraspecies comparison A comparison between the PHPT1 immunohisto- chemical staining results in mouse and human tissues revealed a similar expression pattern in the majority of tissues, e.g. strong expression in the epithelium and a moderate expression level in heart and sceletal muscle cells and some glandular cells. The PHPT1 expression in different epithelial tissues was strong both in adult and embryonic mouse. PHPT1 was detected in the simple columnar epithelial tissue that covers the digestive tubules (such as intestine and stomach), the pseudostratified columnar ciliated epithelium of the lung bronchia, and the simple cuboidal epithelium of kidney tubules (Figure 2A and B and Table I). PHPT1 was also clearly detected in both stratified and non-stratified squamous epithelium in e.g. adult human skin and oesophagus. PHPT1 was not detectable in most