Six candidate hRG gene co-expression modules (Fig. 1d) were superimposed on three independent gene expression data sets generated from laser-microdissected samples from prenatal human cortex: ABI.1 (ref. 13) (GW17), ABI.2 (ref. 13) (GW18), and Fietz et al. (ref. 12) (GW15–18) (as listed in Extended Data Table 1). The characteristic expression patterns of the superimposed modules were summarized by singular value decomposition; the first principal component (PC1) for each module in each data set is shown. In all cases, PC1 revealed substantially higher expression levels for these genes in germinal zones (VZ, ISVZ, and OSVZ, highlighted in red) versus non-germinal zones (IZ, SP, ICP, OCP, CP, MZ, SG). Permutation analysis indicated that the per cent variance explained (VE) by PC1 of each superimposed module was significantly greater than expected by chance (n = 10,000 permutations). CP, cortical plate; CTX, cortex; GW, gestational week; ICP, inner cortical plate; ISVZ, inner subventricular zone; IZ, intermediate zone; MZ, marginal zone; OCP, outer cortical plate; OSVZ, outer subventricular zone; SG, subpial granular layer; SP, subplate; VZ, ventricular zone.