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Little is known about the association between the atypical adenomatous hyperplasia (AAH)-adenocarcinoma in situ sequence of the lung and endoplasmic reticulum-stress responders such as ATF6, XBP1, and GRP78. Using stored tissues, we examined (i) the percentage of a splicing form (active form) of XBP1 messenger RNA in normal lung tissue (NLT) and adenocarcinoma (ACA; using reverse-transcription polymerase chain reaction); (ii) ATF6 and GRP78 protein expressions in NLT and ACA (using Western blotting analysis); (iii) ATF6, XBP1, and GRP78 protein expressions in NLT, AAH, and ACA, including some adenocarcinoma in situ (using immunohistochemistry); and (iv) the incidence of nuclear translocation of the 3 proteins in these lesions. The percentage of the splicing form of XBP1 messenger RNA showed a borderline difference between NLT and ACA (P = .068). In the Western blotting analysis, the nuclear fractions of ATF6 (including the active form) and GRP78 proteins were higher in ACA than in NLT. In the immunohistochemistry, the values obtained for the incidence of the nuclear translocation of ATF6, XBP1, and GRP78 proteins were as follows, respectively: 13.3%, 2.2%, and 0.5% in low-grade AAH; 37.9%, 2.3%, and 2.2% in high-grade AAH; and 47.2%, 10.6%, and 4.4% in ACA. A significant difference was detected between low-grade AAH and ACA for ATF6. In terms of nuclear translocation, high-grade AAH seemed intermediate between low-grade AAH and ACA. These results support endoplasmic reticulum-stress responses, such as nuclear translocation of these 3 proteins (including their active forms), being in parallel with the progression of the adenoma-carcinoma sequence in the lung.