TK1 inactivation relieves the early S-phase RS in dCK^−/− developing B and erythroid cells. (A) Representative examples of B cell development staining of BM samples from WT, dCK^−/−, and dCK^−/−;TK1^−/− DKO mice. IgM and B220 staining of whole BM cells identify Hardy fraction A-D (B220⁺, IgM⁻) and Hardy fraction E-F (B220⁺, IgM⁺) populations. Hardy fraction A-D cells are sub-gated using CD43 and CD19 expression to identify Hardy Fraction A (CD43^hi, CD19⁻), B-C (CD43^hi, CD19⁺), and D (CD43^lo, CD19⁺). Hardy fraction B-C cells are then analyzed for cell cycle position and pH2A.X expression. Plots are representative of n = 3 mice/genotype. (B and C) Quantification of percentage of total BM cells that are phenotypically Hardy fraction A-D (B), and Hardy fraction B-C (C) populations from WT, dCK^−/−, and DKO mice. Data are mean values ±SEM for n = 3/genotype; *, P < 0.04; **, P < 0.01. (D) Representative examples of pH2A.X detection in EryA cells from WT, dCK^−/−, TK1^−/−, and DKO mice. n = 4 mice/genotype. (E) Representative images of spleens from WT, dCK^−/−, TK1^−/−, and DKO mice. (F) dCTP and dTTP pool measurements from nucleated BM cells from WT, dCK^−/−, TK1^−/−, DKO mice. Data are mean values ±SEM. n = 4 mice/genotype. *, P < 0.001.