Hapln1b overexpression inhibits cmyb through interactions mediated by the link domains. (A) Schematic representation of WT hapln1b and a truncated version that lacks exons 4 and 5 (hapln1bΔ4Δ5). (B) ISH expression of cmyb in NI control embryos, hapln1b mRNA–injected embryos and hapln1bΔ4Δ5-injected embryos at 36 hpf. ISH expression of runx1 in control MO (ctl MO), hapln1b MO (h1b MO), hapl1b MO, and hapln1bΔ4Δ5 mRNA–injected (h1b MO and h1bΔ4Δ5), and hapl1b MO and hapln1b full-length mRNA (h1bMO and h1b)–injected embryos at 28 hpf. (C) Summary of proposed model. In the wild-type situation, hapn1b helps kitlgb to interact with kitb in the AGM to permit runx1 expression and HSPC formation. Loss of hapln1b results in a loss of kitlgb interaction with kitb, no runx1 expression, and no HSPCs forming. Overexpression of hapln1b results in a dense ECM and reduced kitlgb interaction with kitb, impairing HSPC emergence and survival. HE, hemogenic endothelium; NI, non-injected.