There is compelling evidence to suggest that serous tubal intraepithelial carcinoma (STIC) is the likely primary site for the development of many pelvic high-grade serous carcinomas (HGSCs). Identifying molecules that are upregulated in STIC is important not only to provide biomarkers to assist in the diagnosis of STIC but also to elucidate our understanding of the pathogenesis of HGSC. In this study, we performed RNA sequencing to compare transcriptomes between HGSC and normal fallopian tube epithelium (FTE), and we identified LAMC1 encoding laminin γ1 as one of the preferentially upregulated genes associated with HGSC. Reverse transcription polymerase chain reaction further validated LAMC1 upregulation in HGSC as compared with normal FTE. Immunohistochemical analysis was performed on 32 cases of concurrent HGSC and STIC. The latter was diagnosed on the basis of morphology, TP53 mutations, and p53 and Ki-67 immunohistochemical patterns. Laminin γ1 immunostaining intensity was found to be significantly higher in STIC and HGSC compared with adjacent FTE in all cases (P<0.001). In normal FTE, laminin γ1 immunoreactivity was predominantly localized in the basement membrane or on the apical surface of ciliated cells, whereas in STIC and HGSC cells, laminin γ1 staining was diffuse and intense throughout the cytoplasm. More importantly, strong laminin γ1 staining was detected in all 13 STICs, which lacked p53 immunoreactivity because of null mutations. These findings suggest that the overexpression of laminin γ1 immunoreactivity and alteration of its staining pattern in STICs can serve as a useful tissue biomarker, especially for those STICs that are negative for p53 and have a low Ki-67 labeling index.