Culture of hESCs

hESCs used in this study include HES-3 human ES cells and H9 human ES cells (WiCell). Genetic modification of the ISL1 locus to generate ISL1-null hESCs was performed on HES-3 cells by applying CRISPR/Cas9 with the same gRNAs used in NHP blastocysts. Two ISL1-knockout cell lines were generated, named ISL1-null_c15 and ISL1-null_c51, and genotyped (Supplementary Fig. ^1a). All cell lines were validated as karyotypically normal by Cell Guidance Systems (UK) (Supplementary Fig. ^9a). Mycoplasma contamination test was performed regularly as negative. hESCs were maintained in a standard feeder-free culture system using mTeSR1 medium on 1% Matrigel or Essential 8 medium on 1% vitronectin. Cells were passaged every 4–5 days and visually examined during each passage to ensure the absence of spontaneous differentiation. Work with hESCs was carried out according to Swedish legislation following the recommendations of the Swedish National Council on Medical Ethics.