Where sections of the genome are missing. This can be short sections only a few bases long, or large tracts of DNA several kilobases long.
Use in clinical context
Deletions of sections of the genome can vary in size and have a range of effects. The size of deletion, however, does not necessarily correspond to the size of the effect.
A single base deletion within a gene can change the entire reading frame and therefore every subsequent amino acid in the protein, thus changing its function. A change in the reading frame can also introduce a stop codon earlier in the sequence, which means the protein will be shorter than it should be, again affecting its function.
A deletion of a large tract of non-coding DNA may or may not have an effect on phenotype. Some non-coding DNA regions have important regulatory functions, while others can be deleted with little apparent impact on phenotype.