Class I human histocompatibility (HLA) surface antigens encoded by alleles on locus B of the HLA complex. The hla-g antigens are considered non-classical class I antigens due to their distinct tissue distribution which differs from HLA-A; HLA-B; and hla-c antigens. Note that several isoforms of hla-g antigens result from alternative splicing of messenger RNAs produced from the HLA-G*01 allele.