<i>roX1</i> RNA is needed to sustain endogenous <i>roX1</i> transcription in males.

<p>X chromosomes from neighboring cells display a mosaic pattern in which the MSL complex either succeeded (arrowhead) or failed (arrow) to paint the X from <i>roX2</i>; <i>msl2</i>; <i>H83M2</i> female (A) and male (B) salivary glands. (C) Endogenous <i>roX1</i> and <i>H83roX1Δ39</i> transcripts (Orange) and antisense riboprobe recognizing only full length <i>roX1</i> (green). Whole mount <i>roX1</i> FISH using the internal probe on salivary glands from (D) wild type male, (E) wild type female, (F) <i>roX1⁺ roX2⁻</i>/Y; <i>msl2</i>; <i>H83M2</i> mosaic male, (G) <i>roX1⁺roX2⁻</i>/Y; <i>msl2; H83M2 H83-roX1Δ39</i>/+ male, (H) <i>roX1⁻ roX2 ⁻</i>/Y; <i>msl2; H83M2 H83-roX1Δ39</i>/+ male. The X chromosomes in G are fully painted in all cells with MSL complex relying upon <i>roX1-Δ39</i> RNA (<a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1002564#pgen.1002564.s001" target="_blank">Figure S1D</a>), but the truncated <i>roX1</i> RNA is not recognized by the internal probe.