Cre DNA and mRNA injection (Figure 3A–C and Figure 4—figure supplement 2B and C) or tamoxifen treatment (Figure 4B–H) was performed on embryos prior to genotyping PCR. Imaging of embryos (Figures 3B and 4C) and screening of the tcf21:DsRed2 transgene (Figure 4G) or GFP+ embryos after Cre DNA injection (Figure 3A–C) were also performed on anesthetized live embryos prior to the PCR genotyping using an epifluorescence microscope.