A method of preparing a deoxyribonucleic acid -enriched fraction from a cancer patient, the method comprising: (a) extracting DNA from a fluid sample of blood plasma or blood serum of the cancer patient to obtain cell-free DNA (cfDNA) fragments; (b) producing, from the extracted cfDNA fragments of (a), a fraction of DNA with fragments of circulating tumor DNA (ctDNA), wherein the fraction of DNA with fragments of ctDNA is produced by (i) contacting the cfDNA fragments of (a) with a plurality of oligonucleotides, wherein each oligonucleotide comprises a nucleic acid sequence that is capable of hybridizing to a DNA fragment comprising one of a subset of a set of tumor-specific somatic mutations, wherein the set of tumor-specific somatic mutations is determined by: sequencing genomic DNA from tumor sample obtained from the cancer patient and sequencing genomic DNA from a non-tumor sample obtained from the cancer patient, thereby obtaining sequences of genomic DNA from the tumor sample and sequences of genomic DNA from the non-tumor sample; aligning the sequences of genomic DNA from the tumor sample to a reference human genome that is not from the patient and aligning the sequences of genomic DNA from the non-tumor sample to the reference genome that is not from the patient, thereby obtaining a set of tumor-specific somatic mutation sequences that are present in the sequences of genomic DNA from the tumor sample but not present in the sequences of genomic DNA from the non-tumor sample, and (ii) selectively enriching the extracted cfDNA fragments of (a) with the plurality of oligonucleotides, wherein the DNA fraction after (b) is enriched with DNA fragments comprising one or more of the subset of tumor-specific somatic mutations. (a) extracting DNA from a fluid sample of blood plasma or blood serum of the cancer patient to obtain cell-free DNA (cfDNA) fragments; (b) producing, from the extracted cfDNA fragments of (a), a fraction of DNA with fragments of circulating tumor DNA (ctDNA), wherein the fraction of DNA with fragments of ctDNA is produced by (i) contacting the cfDNA fragments of (a) with a plurality of oligonucleotides, wherein each oligonucleotide comprises a nucleic acid sequence that is capable of hybridizing to a DNA fragment comprising one of a subset of a set of tumor-specific somatic mutations, wherein the set of tumor-specific somatic mutations is determined by: sequencing genomic DNA from tumor sample obtained from the cancer patient and sequencing genomic DNA from a non-tumor sample obtained from the cancer patient, thereby obtaining sequences of genomic DNA from the tumor sample and sequences of genomic DNA from the non-tumor sample; aligning the sequences of genomic DNA from the tumor sample to a reference human genome that is not from the patient and aligning the sequences of genomic DNA from the non-tumor sample to the reference genome that is not from the patient, thereby obtaining a set of tumor-specific somatic mutation sequences that are present in the sequences of genomic DNA from the tumor sample but not present in the sequences of genomic DNA from the non-tumor sample, and (ii) selectively enriching the extracted cfDNA fragments of (a) with the plurality of oligonucleotides, (i) contacting the cfDNA fragments of (a) with a plurality of oligonucleotides, wherein each oligonucleotide comprises a nucleic acid sequence that is capable of hybridizing to a DNA fragment comprising one of a subset of a set of tumor-specific somatic mutations, wherein the set of tumor-specific somatic mutations is determined by: sequencing genomic DNA from tumor sample obtained from the cancer patient and sequencing genomic DNA from a non-tumor sample obtained from the cancer patient, thereby obtaining sequences of genomic DNA from the tumor sample and sequences of genomic DNA from the non-tumor sample; aligning the sequences of genomic DNA from the tumor sample to a reference human genome that is not from the patient and aligning the sequences of genomic DNA from the non-tumor sample to the reference genome that is not from the patient, thereby obtaining a set of tumor-specific somatic mutation sequences that are present in the sequences of genomic DNA from the tumor sample but not present in the sequences of genomic DNA from the non-tumor sample, and sequencing genomic DNA from tumor sample obtained from the cancer patient and sequencing genomic DNA from a non-tumor sample obtained from the cancer patient, thereby obtaining sequences of genomic DNA from the tumor sample and sequences of genomic DNA from the non-tumor sample; aligning the sequences of genomic DNA from the tumor sample to a reference human genome that is not from the patient and aligning the sequences of genomic DNA from the non-tumor sample to the reference genome that is not from the patient, thereby obtaining a set of tumor-specific somatic mutation sequences that are present in the sequences of genomic DNA from the tumor sample but not present in the sequences of genomic DNA from the non-tumor sample, and (ii) selectively enriching the extracted cfDNA fragments of (a) with the plurality of oligonucleotides,