“…It was only in syncytial cytoplasm that hCG was detected without expansion to cytotrophoblast. In regard to a developmental mechanism of trophoblast-like cells in the lung, the following three are acceptable for consideration; i) malignant change of normal trophoblast by transplantation to the lung (10,17,19), ii) primary choriocarcinoma of the lung (21), and iii) trophoblastic differentiation of carcinoma cells in the lung (2,6). Authors conducted an investigation to distinguish those three by the difference of localization and/or distribution of hCG-3.…”