Immunohistochemical analysis for mast-cell tryptase in human fetal BM sections after engraftment of unsorted WM cells. A fetal bone engrafted with IgMκ WM cells was retrieved from murine host after detection of rising levels of human paraprotein in mouse serum and then examined for the presence of mast cells as assessed by immunostaining for mast-cell tryptase. Original magnification, × 400.