A composition that specifically binds indoleamine 2,3-dioxygenase 1 (IDO1) comprising a first ligand having affinity for an epitope on IDO1, wherein the ligand is a cyclic peptide comprising a 3-10 amino acid sequence of D-amino acids and artificial amino acids and wherein IDO1 comprises an active site, and: i) wherein the epitope comprises the amino acid sequence GFWEDPKEFAGGSAGQSSVFQ, and the ligand comprises an amino acid sequence selected from the group consisting of: a. X1rf(Me-Trp)X2, wherein X1 and X2 are independently D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; b. nsfr(Me-Trp); and c. wyrX3y, wherein X3 is D-Ala or is not present; or ii) wherein the epitope comprises the amino acid sequence LPPILVYADCVLANWKKKDPNK, and the ligand comprises an amino acid sequence selected from the group consisting of: d. rys(Me-Trp)r; e. X4lf(Me-Trp)(F-Phe), wherein X4 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; f. nlw(Me-Trp)r; and g. sX5ww(F-Phe), wherein X5 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; or iii) wherein the epitope comprises the amino acid sequence NKPLTYENMDVLFSFR, and the ligand comprises an amino acid sequence selected from the group consisting of: h. rffyl; and i. nsh(F-Phe)r. i) wherein the epitope comprises the amino acid sequence GFWEDPKEFAGGSAGQSSVFQ, and the ligand comprises an amino acid sequence selected from the group consisting of: a. X1rf(Me-Trp)X2, wherein X1 and X2 are independently D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; b. nsfr(Me-Trp); and c. wyrX3y, wherein X3 is D-Ala or is not present; or a. X1rf(Me-Trp)X2, wherein X1 and X2 are independently D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; b. nsfr(Me-Trp); and c. wyrX3y, wherein X3 is D-Ala or is not present; ii) wherein the epitope comprises the amino acid sequence LPPILVYADCVLANWKKKDPNK, and the ligand comprises an amino acid sequence selected from the group consisting of: d. rys(Me-Trp)r; e. X4lf(Me-Trp)(F-Phe), wherein X4 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; f. nlw(Me-Trp)r; and g. sX5ww(F-Phe), wherein X5 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; or d. rys(Me-Trp)r; e. X4lf(Me-Trp)(F-Phe), wherein X4 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; f. nlw(Me-Trp)r; and g. sX5ww(F-Phe), wherein X5 is D-Ala, D-Arg, D-Asn, D-Asp, D-Glu, Gly, D-His, D-Leu, D-Lys, D-Phe, D-Pro, D-Ser, D-Thr, D-Trp, D-Tyr, or D-Val or is not present; iii) wherein the epitope comprises the amino acid sequence NKPLTYENMDVLFSFR, and the ligand comprises an amino acid sequence selected from the group consisting of: h. rffyl; and i. nsh(F-Phe)r. h. rffyl; and i. nsh(F-Phe)r. <EOS>