Figure 2A FACS analysis of the percentages of CD44+/CD24−/ESA+ breast CSC-like populations in CDK12-overexpressing ZR-75-30 and HCC-1419 cells and CDK12-siRNA-transfected BT474 and HCC-1954 cells. Top, representative dot plot of CD44-APC versus CD24-PE expression for viable cells (red) and ESA-positive cells (blue) from the viable cells. Bottom, the percentage of CD44+/CD24-/ESA+ cell population (blue dots at lower right quadrant in the plots) was quantified in the indicated cell lines. Mean ± s.d. of three independent samples. P-values were based on two-tailed Student's t-test (ZR-75-30 and HCC-1419 cells) and one-way ANOVA with a post-hoc LSD test (BT474 and HCC-1954 cells). Amp, amplification; non-amp, non-amplification; Trz, trastuzumab. B ALDEFLUOR assay for analysing cell population with a high ALDH activity in the indicated cell lines. Cells either treated with ALDH inhibitor DEAB (with DEAB; negative control) or untreated (without DEAB) were incubated with ALDEFLURO substrate (BAAA). In the FACS analysis, the percentage of ALDEFLUOR-positive population was determined based on the shift of fluorescent cells seen in the dot plots in the absence of DEAB. Mean ± s.d. . P-values were calculated based on a two-tailed Student's t-test cell line information was indicated as follows: a/a, CDK12-amplified/HER2-amplified cells; n/a, CDK12 non-amplified/HER2-amplified cells S, trastuzumab-sensitive. C The self-renewal of CSCs in the indicated stable cell lines was analysed using a tumorsphere-formation assay. The number of tumorsphere cells (> 100 μm in diameter) was counted after 3, 5, and 7 days. Data represent the mean ± s.d. (n = 3). P-value was calculated based on a two-tailed Student's t-test (ZR-75-30 and HCC-1419 cells) or ANOVA with a post-hoc LSD test (BT474 and HCC-1954 cells). cell line information was indicated as follows: a/a, CDK12-amplified/HER2-amplified cells; n/a, CDK12 non-amplified/HER2-amplified cells; R, trastuzumab-resistant; PR, trastuzumab-partial responsive; S, trastuzumab-sensitive. D Cell growth of the indicated stable cell lines treated with trastuzumab (Trz) or vehicle (veh) as measured by SRB assay. Trastuzumab-sensitive HER2+ breast cancer cell lines (S) and trastuzumab-insensitive HER2+ breast cancer cell lines (partial responsive, PR; resistant, R) were treated with 1 µg/mL or 100 µg/mL trastuzumab, respectively. Data represent the mean ± s.d. P-values were calculated using RM ANOVA with a post-hoc LSD test. cell line information was indicated as follows: a/a, CDK12-amplified/HER2-amplified cells; n/a, CDK12 non-amplified/HER2-amplified cells; R, trastuzumab-resistant; PR, trastuzumab-partial responsive; S, trastuzumab-sensitive. E Analysis of the effect of CDK12 deficiency on the in vivo trastuzumab response of HER2+ breast cancer. Mice were orthotopically xenografted with the indicated HER2+ breast cancer cells and treated with 20 mg/kg trastuzumab. The growth curve of each group was analysed twice weekly for 5 to 6 weeks (n = 8/group; mean ± s.e.m.). P-values were calculated using one-way ANOVA with a post-hoc LSD test for the last day of tumor measurement. cell line information was indicated as follows: a/a, CDK12-amplified/HER2-amplified cells R, trastuzumab-resistant S, trastuzumab-sensitive.