From Reason I am Here:
The first time I tried cilantro I didn’t realize it; I just thought somebody had emptied a bottle of Old Spice on my pizza in an attempt to poison me. Cilantro tastes like soap to approximately 10% of the people who have had their genotype analyzed by 23andMe. The currently accepted explanation is that those of us who passionately despise cilantro were born with a genetic variant known as a single-nucleotide polymorphism (or SNP, pronounced ‘snip’).
The genome has 3 billion nucleotides (the building blocks, known as A, C, G and T), and 10 million of them are thought to be SNPs. That means that a significant percentage of the population has one letter in a specific location (an A, for example) and everyone else has a different letter at that location. The cilantro SNP is called rs72921001, and apparently, its genomic location lays close to a cluster of olfactory receptor genes that includes OR6A2, the gene most likely to be alerting our brain about the presence of cilantro.