Frequency of expression. We assessed the frequency of expression of PDGFs semi-quantitatively by the following method. Five fields of each PDGF isoform, in which abnormal angiogenesis were detected, were randomly selected with a microscope. The PDGF-positive mononuclear cells were counted. We observed all 7 cases and performed the counting using two observers to reduce bias. One observer, who was blind to the patients’ clinical and pathological information, evaluated the results of the immunohistochemical staining. The ratios of PDGF-positive cells to total cells in each field were calculated and were statistically analyzed using Steel-Dwass tests with JMP Pro 10 (SAS Institute Inc., Cary, NC, USA). Statistical analysis revealed that PDGF-C and D showed higher frequency of expression in the PN specimens than did PDGF-A and B. The difference was statistically significant (*p < 0.0001, Steel-Dwass test).