Reporter Tethering assay and GFP Quantitation.

Tethering assay quantitation was performed as described (16). Briefly, GFP intensity was measured using FIJI at one distal region and one proximal region within the progenitor zone, and the ratio of GFP intensities in the distal and proximal regions was calculated in Microsoft Excel. Samples from two independent replicates for each genotype were analyzed, and background was normalized to a wild-type control that had no GFP. Sample sizes are reported in the Fig. 4 legend.