from gently squashed discs were counted (Figure S1) N = cells counted. Genotypes: (1–2) roX1 + roX2+;msl2;[H83-M2]/+, (3–4) roX1+roX22;msl2;[H83-M2]/+, (5) roX1+roX2 2 /Y;msl2/+;[H83- M2]/+ male, (6) roX1+roX22;msl2/+; [NOPU-M2][H83-M2]/+ female. (PDF) Figure S5 MSL2 immunostaining of embryos reveals that 100% of the cells expressed [H83-MSL2]. (A–A0) A wildtype embryo at the end of germband extension showing endogenous MSL2 2 expression detected in 100% of the cells (B–B0) A roX2 ; msl2; [H83M2] embryos, expected to display a mosaic pattern of dosage compensation in imaginal discs and salivary glands by 3rd instar, shows no signs of mosaicism during early embryogenesis. (PDF) Figure S6 roX1 in situ hybridization of embryos reveals that 100% of the cells successfully establish dosage compensation. (A– A0) A wildtype embryo at about 20 hr AEL, has roX1 expression 2 detected in 100% of the cells (B–B0) A similarly stage roX2 ; msl2; [H83M2] embryos displaying no signs of mosaicism. 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