A composition comprising a homogeneous population of polypeptides comprising an extended recombinant polypeptide (XTEN), wherein at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% of individual polypeptide molecules in said population have an identical sequence length, wherein the XTEN is characterized in that a. the XTEN comprises at least 36 to 3000 amino acid residues; b. the sum of glycine, alanine, serine, threonine, glutamate and proline residues constitutes more than about 90% of the total amino acid residues of the XTEN; c. the XTEN sequence is substantially non-repetitive such that (i) the XTEN sequence contains no three contiguous amino acids that are identical unless the amino acids are serine, and (ii) at least 80% of the XTEN sequence consists of non-overlapping sequence motifs, each of the sequence motifs comprising at least 9 to 14 amino acid residues and wherein each of the motifs consists of 4 to 6 types of amino acids selected from glycine, alanine, serine, threonine, glutamate and proline, wherein any two contiguous amino acid residues do not occur more than twice in each of the sequence motifs; d. the XTEN sequence has greater than 90% random coil formation as determined by GOR algorithm; e. the XTEN sequence has less than 2% alpha helices and 2% beta-sheets as determined by Chou-Fasman algorithm; f. the XTEN sequence lacks a predicted T-cell epitope when analyzed by TEPITOPE algorithm, wherein the TEPITOPE algorithm prediction for epitopes within the XTEN sequence is based on a score of -6; and wherein the polypeptides of the population have the configuration of formula II: - -(XTEN)- - II wherein g. HS is a sequence having at least 90% sequence identity to a sequence selected from the group consisting of the sequences set forth in Table 10; h. AT1 is a first affinity tag having an amino acid sequence exhibiting binding affinity for a chromatography substrate selected from the group consisting of hydrophobic interaction chromatography substrate, cation exchange substrate, anion exchange substrate, immobilized metal ion affinity chromatography (IMAC) substrate, and immobilized antibody substrate; i. CS1 is a first amino acid sequence capable of being cleaved by trypsin; j. CS2 is a second amino acid sequence capable of being cleaved by trypsin; and k. XTEN is the extended recombinant polypeptide. a. the XTEN comprises at least 36 to 3000 amino acid residues; b. the sum of glycine, alanine, serine, threonine, glutamate and proline residues constitutes more than about 90% of the total amino acid residues of the XTEN; c. the XTEN sequence is substantially non-repetitive such that (i) the XTEN sequence contains no three contiguous amino acids that are identical unless the amino acids are serine, and (ii) at least 80% of the XTEN sequence consists of non-overlapping sequence motifs, each of the sequence motifs comprising at least 9 to 14 amino acid residues and wherein each of the motifs consists of 4 to 6 types of amino acids selected from glycine, alanine, serine, threonine, glutamate and proline, wherein any two contiguous amino acid residues do not occur more than twice in each of the sequence motifs; d. the XTEN sequence has greater than 90% random coil formation as determined by GOR algorithm; e. the XTEN sequence has less than 2% alpha helices and 2% beta-sheets as determined by Chou-Fasman algorithm; f. the XTEN sequence lacks a predicted T-cell epitope when analyzed by TEPITOPE algorithm, wherein the TEPITOPE algorithm prediction for epitopes within the XTEN sequence is based on a score of -6; and g. HS is a sequence having at least 90% sequence identity to a sequence selected from the group consisting of the sequences set forth in Table 10; h. AT1 is a first affinity tag having an amino acid sequence exhibiting binding affinity for a chromatography substrate selected from the group consisting of hydrophobic interaction chromatography substrate, cation exchange substrate, anion exchange substrate, immobilized metal ion affinity chromatography (IMAC) substrate, and immobilized antibody substrate; i. CS1 is a first amino acid sequence capable of being cleaved by trypsin; j. CS2 is a second amino acid sequence capable of being cleaved by trypsin; and k. XTEN is the extended recombinant polypeptide.