Stool serotonin measured during bone marrow transplant and skin biopsy samples from patients with FA and normal matched control subjects stained for serotonin and TPH1. (A) Note the logarithmic scale. All patients undergoing bone marrow transplant display a marked decrease in stool serotonin (388.98 ng/mL [336.61-743.74] vs 113.97 ng/mL [45.76-183.10]; P = .0001), including those with FA (284.98 ng/mL [81.53-419.38] vs 61.20 ng/mL [15.62-111.70]; P = .0026). Immunohistochemical staining for serotonin of skin biopsy samples at original magnification 400× from 3 different patients with FA (B-D) display marked cytoplasmic positivity within the epidermis at the level of the basal keratinocytes. (E-G) Identical staining of normal human skin biopsy samples was negative for serotonin. Immunohistochemical staining for TPH1 of skin biopsy samples from 3 patients with FA (H-J) and 3 normal human skin biopsy samples (K-M) display focal positivity within the keratinocytes of the epidermis. Original magnification 400× for panels B through M. Both positive and negative control tissue stained appropriately.