Required for the normal development of the forebrain, eyes and other anterior structures such as the olfactory placodes and pituitary gland. Possible transcriptional repressor. Binds to the palindromic PIII sequence, 5'-AGCTTGAGTCTAATTGAATTAACTGTAC-3'. HESX1 and PROP1 bind as heterodimers on this palindromic site, and, in vitro, HESX1 can antagonize PROP1 activation. {ECO:0000250|UniProtKB:Q61658, ECO:0000269|PubMed:26781211}. This is the function of HESX1 (HESX homeobox 1, Ensembl gene identifier ENSG00000163666).