(A) Low magnification (left) and high magnification (right) images of FOXP2 (yellow) immunohistochemical labeling of E14.5 forebrain reveals absence of expression in the dorsal pallium, whereas the developing striatum is robustly labeled at this timepoint (N = 5). (B) Images of FOXP2 immunolabeling at E16.5 demonstrates the presence of FOXP2 expression within the deep layers of the developing cortical plate (N = 3). (C) Retrograde labeling of layer 6 corticothalamic neurons (magenta) by injection of CTB into the ventrobasal thalamus, combined with FOXP2 (yellow) immunohistochemistry at P14. (D) Corticocortical neurons (cyan) labeled by injection of CTB into the ipsilateral primary motor cortex, combined with FOXP2 (yellow) immunohistochemistry at P14. White arrows denote retrogradely labeled projection neurons. (E) Quantification of the percentages of retrogradely labeled corticothalamic (N = 4 mice) and corticocortical (N = 5) neurons that express FOXP2. (F) FOXP2 (yellow) immunohistochemistry in sections of P0, P7, and P14 somatosensory cortex from Ntsr1-cre; tdTomato mice (tdTomato is magenta); white asterisks denote relatively low-level expression in layer 5 at P0. (G) Quantification of the percentages of tdTomato-positive neurons that express FOXP2 at each age (P0, N = 3; P7, N = 3; P14, N = 3). (H) FOXP2 (magenta) immunohistochemistry in sections of P0, P7, and P14 somatosensory cortex from MetGFP (green) mice. Cyan arrowheads denote sparse FOXP2+ and GFP+ double-labeled cells localized to layer 6B/subplate. (I) Quantification of the percentages of GFP+ neurons that co-express Foxp2 at each age (P0, N = 3; P7, N = 4; P14, N = 3). (J) FOXP2 (yellow) colocalizes with PCP4 (magenta), but not ppCCK (cyan) (N = 3). Scale bars: 500 µm, A, B low magnification; 100 µm A, B high magnification; 50 µm C, D, F, H and J.