Fig. S1. PV⁺ interneurons are absent in the ventrolateral cortex at early postnatal ages. (A-D) Ventrolateral areas of the cortex of WT mice do not exhibit PV immunoreactive cells (green in A,C) at P0 or P7. GABA (red in A-D) and SST (green in B,D) immunoreactive cells are present at P0 and P7 in the ventrolateral cortex. Scale bar: 50 m.

Fig. S2. BMP4 increases neuronal differentiation of MGE progenitors. (A-C) MGE progenitors differentiated in the presence of low bFGF (0.5 ng/ml, Control) or with BMP4 (20 ng/ml) or noggin (250 ng/ml). Immunostaining for GABA (red) and βIII-tubulin (green), with DAPI-labeled nuclei in blue. (D) Quantification of the different immunopositive populations. Note that the GABAergic interneuron population (βIII-tubulin⁺/GABA) is ∼65% of the total neuronal population (βIII-tubulin⁺) under all three conditions. Scale bar: 100 m.

Fig. S3. PV⁺ cells in cortical cultures after BMP4 treatment are GABAergic. (A-B′′′) Cortical cultures treated with BMP4 for 96 hours immunostained for PV (red; A′′,B′′), GABA (green; A′) or VGLUT1 (green; B′), with DAPI-labeled nuclei in blue (A,B). PV⁺ cells (arrowheads) coexpress GABA (A-A′′′) but do not express VGLUT1 (B-B′′′). Note the presence of VGLUT⁺ cells (arrows) lacking PV expression (B-B′′′). Scale bar: 50 m.

Fig. S4. BMP4 causes PV expression in OLIG1 lineage-derived interneuron precursors. (A-B′′′) Cortical cultures from P0 Olig1^cre/+; ROSA^GFP/+ mice in control (A-A′′′) or BMP4-treated (B-B′′′) conditions, immunostained for GFP (green) and PV (red), with DAPI-labeled nuclei in blue. (C,D) Quantification of the fraction of PV⁺ cells labeled with GFP (C) and the fraction of GFP⁺ cells that were PV⁺ (D) under the two conditions. (E-E′′′) A low magnification view of the culture after BMP4 treatment showing multiple PV⁺/GFP⁺ cells. **P<0.01, ***P<0.001, Student’s t-test; n.d, not detected. Scale bar: 50 m.

Fig. S5. Strong Bmp4 overexpression in the hippocampus of Nse-Bmp4 mice. (A-C) Bmp4 transcript as revealed by in situ hybridization in E15.5 WT (A) and Nse-Bmp transgenic (B,C) embryos. Note the relatively weak transgene expression in the developing cortex (B). (D-F) Bmp4 expression in P0 WT (D) and Nse-Bmp transgenic (E,F) pups. Bmp4 transcript is detected in the primary motor cortex but is barely visible in the hippocampus of WT animals (D). However, Nse-Bmp transgenic animals show consistent expression throughout the cortical layers and even stronger expression in the hippocampus (E). Strong subpallial expression is also visible in the Nse-Bmp transgenic animals at P0 (F). Scale bar: 250 m.

Fig. S6. SST⁺ interneurons are not exposed to strong BMP4 signaling in vivo. (A-B′) BMPR1A (red) is expressed at high levels in SST⁺ (green) interneurons at P0 (A-A′) and P7 (B-B′). (C-D′) Subpopulations of SST⁺ interneurons at P0 (C-C′) and P7 (D-D′) express BMPR1B at levels much lower than BMPR1A. Note the higher expression levels of BMPR1A and BMPR1B in the pyramidal cells in the cortex at P7. (E-F′) SST⁺ interneurons rarely display strong nuclear pSMAD1/5/8 staining at P0 (E-E′) and P7 (F-F′). Scale bar: 50m.

Fig. S7. SST⁺ interneurons in the ventrolateral cortex are increased in Bmpr1a/b double mutant mice at P0. (A,B) The piriform area of Bmpr1a/b double mutant mice (B) displays an increase in the number of SST⁺ interneurons (red) compared with control mice (A). (C,D) Quantification of SST⁺ cell numbers in the somatosensory cortex (C) and piriform area (D). *P<0.05, Student’s t-test. Scale bar: 50m.

Fig. S8. Loss of BMP type I receptors does not affect the number of GABAergic interneurons in the cortex. (A,B) Bmpr1a/b double null mice (B) have similar numbers of GABAergic interneurons (red) compared with their control littermates (A) in the somatosensory cortex at P21. Scale bar: 50m.