Zebrafish nicd1a overexpression results in an increase of cpa5-positive embryonic mast cells, which can be reversed by Notch inhibition using CpdE. WISH using a digoxigenin-labeled RNA antisense probe to zebrafish cpa5 was performed at different time points in hsp70::GAL4;UAS::nicd1a embryos heat shocked at 13 to 15 hpf. Bar graphs represent the percent of embryos categorized by WISH expression levels. (A) Overexpression of nicd1a results in an increase in the number of cpa5-positive mast cells at 24 hpf (n = 22) and 28 hpf (n = 49) compared with wild-type embryos (n = 20 and 52, respectively). (B) Treatment of hsp70::GAL4;UAS::nicd1a transgenic embryos (n = 43) with the Notch inhibitor CpdE for 8 hours (22-30 hpf) restores wild-type cpa5 expression levels at 25μM (n = 39) with further reduced expression at 50μM (n = 41) compared with wild-type embryos (n = 47). Insets: Magnified view of the area of interest (5× objective, MZ6 microscope, Leica).