A method of detecting loss of heterozygosity in HLA alleles is provided. The method can include accessing a trained machine-learning model, which was trained using a training data set that included at least a training data set that includes an adjusted B allele frequency that represents a ratio between a first B allele frequency of heterozygous alleles in the tumor sample that correspond to the genomic region and a second B allele frequency of heterozygous alleles in the genomic region and associated with one or more control samples. The method can also include using the machine-learning model to generate a result corresponding to a probability of whether a loss of heterozygosity exists in an HLA allele identified in the biological sample of the particular subject by processing the sequence data using the machine-learning model.