Immunohistochemical analyses for Id-1 in mammary glands. All tissue sections were treated with the primary antibody (except (B), which was incubated with both the primary antibody and the blocking peptide) and processed as described in the text. (A), (B) Adult nulliparous wild type mice (FVB), (C) normal human mammary gland, (D) adult nulliparous wild type mice (129Sv, C57BL/6) corresponding to the strain of Id-1 null mutant mice, (E) arrows showing the vascular endothelial cells, and (F) vascular endothelial cells (arrows) in an artery (A) and a vein (V) in mammary glands of wild type mice. Original magnification: (A)-(E) 400 ×. Insets of (A) and (E), higher magnification (630 ×).