Regulator of endoplasmic reticulum secretion that acts as a key determinant of brain size (PubMed:33122427). Required for secretion of extracellular matrix proteins (PubMed:33122427). Required for correct brain development by depositing sufficient extracellular matrix proteins for tissue integrity and the proliferation of neural progenitors (PubMed:33122427). Acts as a regulator of the unfolded protein response (UPR) (By similarity). {ECO:0000250|UniProtKB:Q9CR20, ECO:0000269|PubMed:33122427}. This is the function of IER3IP1 (immediate early response 3 interacting protein 1, Ensembl gene identifier ENSG00000134049).