Additionally, the measurement of immunofluorescence (IF) showed that the level of FNDC5/Ir expression in normal breast cells (Me16c) was significantly lower (mean 18.54 ± 4.50 SD) compared to the level in the cells of all BC lines (MCF-7—mean 125.60 ± 36.63 SD, p < 0.0001; MDA-MB-231 mean 37.81 ± 20.47 SD, p = 0.0327; MDA-MB-468 mean 74.13 ± 15.40 SD, p < 0.0001). Here, FNDC5 is linked to breast cancer.