Indicate whether each of the following descriptions better applies to class I (1) or class II (2) MHC proteins of vertebrates. Your answer would be a four-digit number composed of digits 1 and 2 only, e.g. 2221.
( ) They mainly present intracellular peptide fragments.
( ) They are found on most nucleated cells.
( ) They present antigens mainly to helper or regulatory T cells.
( ) They are recognized by CD4 co-receptors on helper and regulatory T cells.