Expressed Sequence Tags (ESTs) are tiny portions of genes that can be used to help identify unknown genes within a genome. ESTs are created by sequencing one or both ends of an expressed gene. When working with EST data in the public domain, there is a possibility that the data may contain errors or ambiguities and may also be fragmented (many are <100nt). Additionally, some of the data may contain small segments of expressed contaminant vector of unknown origin. The quality of an assembly depends on the quality of the data added, and some EST data may not meet the quality standards you want.