A recombinant polypeptide comprising an amino acid sequence truncated relative to a full-length human type 21 alpha 1 collagen, and comprising an amino acid sequence according to SEQ ID NO: 76, wherein the amino acid sequence truncated relative to a full-length human type 21 alpha 1 collagen comprises: (i) a truncation at an N-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 600 amino acids, from 50 amino acids to 550 amino acids, from 50 amino acids to 500 amino acids, from 50 amino acids to 450 amino acids, from 50 amino acids to 400 amino acids, from 50 amino acids to 350 amino acids, from 50 amino acids to 300 amino acids, from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen; and (ii) a truncation at a C-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen, and wherein the recombinant polypeptide is monomeric. wherein the amino acid sequence truncated relative to a full-length human type 21 alpha 1 collagen comprises: (i) a truncation at an N-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 600 amino acids, from 50 amino acids to 550 amino acids, from 50 amino acids to 500 amino acids, from 50 amino acids to 450 amino acids, from 50 amino acids to 400 amino acids, from 50 amino acids to 350 amino acids, from 50 amino acids to 300 amino acids, from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen; and (ii) a truncation at a C-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen, and (i) a truncation at an N-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 600 amino acids, from 50 amino acids to 550 amino acids, from 50 amino acids to 500 amino acids, from 50 amino acids to 450 amino acids, from 50 amino acids to 400 amino acids, from 50 amino acids to 350 amino acids, from 50 amino acids to 300 amino acids, from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen; and (ii) a truncation at a C-terminal end relative to a full-length human type 21 alpha 1 collagen, wherein the truncation is from 50 amino acids to 250 amino acids, from 50 amino acids to 200 amino acids, from 50 amino acids to 150 amino acids, or from 50 amino acids to 100 amino acids, relative to a full-length human type 21 alpha 1 collagen, and wherein the recombinant polypeptide is monomeric.