Generation and validation of a novel GH-IRES-Cre mouse line. (A) Schematic of the derivation of the GH-IRES-Cre mice using CRISPER-Cas9 genome editing to insert an IRES-Cre cassette into exon 5 of the GH gene (insertion site marked by an arrow). (B) Validation of somatotroph-specific Cre activity in the GH-IRES-Cre mouse line as performed by observing overlapping GH immunoreactivity and tdTomato fluorescence in pituitaries but not brains of mice harboring a GH-IRES-Cre allele and a Rosa26-lox-STOP-lox-tdTomato transgene. Representative photomicrographs of a pituitary [at 10x magnification (a–c) and 40x magnification (d–f)] and brain [at 10x magnification as a negative control)] (g–l). tdTomato fluorescence (red) (a, d, g, and j); GH immunoreactivity (green) in the same sections (b, e, h, and k); co-localized tdTomato and GH immunoreactivity (yellow) in the same sections (c, f, i, and l); and DAPI nuclear stain (blue) (I and l). Note: In panel B.b, the labels A, I, and P indicate A: anterior lobe; I: intermediate lobe; and P: posterior lobe of the pituitary. n = 4. Scale bars = 100 μm. Expression of (C) GH mRNA and (D) Cre in various tissues. Data are 2-ΔΔCt values relative to the expression of GH or Cre in the pituitary, n = 5.