The invention provides methods for detecting single nucleotide variants in breast cancer, bladder cancer, or colorectal cancer. Additional methods and compositions, such as reaction mixtures and solid supports comprising clonal populations of nucleic acids, are provided. For example, provided here is a method for monitoring and detection of early relapse or metastasis of breast cancer, bladder cancer, or colorectal cancer, comprising generating a set of amplicons by performing a multiplex amplification reaction on nucleic acids isolated from a sample of blood or urine or a fraction thereof from a patient who has been treated for a breast cancer, bladder cancer, or colorectal cancer, wherein each amplicon of the set of amplicons spans at least one single nucleotide variant locus of a set of patient-specific single nucleotide variant loci associated with the breast cancer, bladder cancer, or colorectal cancer; and determining the sequence of at least a segment of each amplicon of the set of amplicons that comprises a patient-specific single nucleotide variant locus, wherein detection of one or more patient-specific single nucleotide variants is indicative of early relapse or metastasis of breast cancer, bladder cancer, or colorectal cancer.