A method of forming a CendR conjugate, the method comprising (a) selecting an amino acid sequence for internalization into a cell, tissue penetration, or both, wherein the amino acid sequence comprises either (i) a CendR element, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), wherein the CendR element is at a C-terminal end of the conjugate and has a C-terminal carboxyl group, or (ii) an activatable CendR element comprising a CendR element and a blocking group, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), and (b) causing a cargo composition to be covalently coupled or non-covalently associated with a protein or peptide, preferably a circular protein or peptide, comprising the selected amino acid sequence, wherein the cargo composition is coupled or associated with the protein or peptide on the N-terminal side of the CendR element, wherein the CendR conjugate comprises the protein or peptide and the coupled or associated cargo composition, wherein the activatable CendR element is formed by covalently coupling the blocking group to the CendR element via a cleavable bond, wherein the bond coupling the blocking group and the CendR element is selected to be cleavable by a protease, wherein cleavage of the bond of the activatable CendR element exposes the CendR element at the C-terminal end of the conjugate, wherein cleavage of the bond makes the CendR element capable of internalization into a cell. (a) selecting an amino acid sequence for internalization into a cell, tissue penetration, or both, wherein the amino acid sequence comprises either (i) a CendR element, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), wherein the CendR element is at a C-terminal end of the conjugate and has a C-terminal carboxyl group, or (ii) an activatable CendR element comprising a CendR element and a blocking group, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), and (i) a CendR element, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), wherein the CendR element is at a C-terminal end of the conjugate and has a C-terminal carboxyl group, or (ii) an activatable CendR element comprising a CendR element and a blocking group, wherein the CendR element has the sequence RPARPAR (SEQ ID NO: 2), RGERPPR (SEQ ID NO: 27), or RVTRPPR (SEQ ID NO: 28), and (b) causing a cargo composition to be covalently coupled or non-covalently associated with a protein or peptide, preferably a circular protein or peptide, comprising the selected amino acid sequence, wherein the cargo composition is coupled or associated with the protein or peptide on the N-terminal side of the CendR element, wherein the CendR conjugate comprises the protein or peptide and the coupled or associated cargo composition, wherein the activatable CendR element is formed by covalently coupling the blocking group to the CendR element via a cleavable bond, wherein the bond coupling the blocking group and the CendR element is selected to be cleavable by a protease, wherein cleavage of the bond of the activatable CendR element exposes the CendR element at the C-terminal end of the conjugate, wherein cleavage of the bond makes the CendR element capable of internalization into a cell.