Alternative splicing is a pervasive process in eukaryotic organisms. More than 90% of human genes have alternatively spliced products, and aberrant splicing has been shown to be associated with many diseases. Current methods employed in the detection of splice variants include prediction by clustering of expressed sequence tags, exon microarray, and mRNA sequencing, all methods focusing on RNA-level information. There is a lack of tools for analyzing splice variants at the protein level. Here, we present SpliceVista, a tool for splice variant identification and visualization based on mass spectrometry proteomics data. SpliceVista retrieves gene structure and translated sequences from alternative splicing databases and maps MS-identified peptides to splice variants. Eukaryotic genes are composed of exonic (protein-coding) and intronic (non-coding) regions. Alternative splicing is a process in which pre-mRNA is cut at junction sites and the resulting exonic sequences are reconnected in different ways to form different versions of mature mRNA. It has been shown that 92% to 94% of human genes can undergo alternative splicing (1, 2). This process plays an essential role in increasing the proteome diversity in eukaryotic organisms. For multiexon mRNAs, different splicing patterns can occur, such as exon skipping (an exon is either included or excluded from the mature mRNAs), alternative 5Ј or 3Ј splicing (exons are spliced in different lengths), or mutually exclusive splicing (exons are selectively spliced to be exclusively present in different splice forms). Alternative splicing is carried out by the spliceosome, which consists of five small nuclear ribonucleoprotein particles, U1, U2, U4, U5, and U6, and more than 150 other proteins (3). Mutations in splicing sites or in the main components of the splicing machinery will affect the genes' splicing patterns and potentially give rise to alternative protein products that might have different conformations, functions, or subcellular locations. Disruption of the splicing machinery has been shown to be associated with many human diseases such as cystic fibrosis, Alzheimer disease, and cancer (4 -6).Large efforts have been put into the identification of gene products generated by alternative splicing. This is a challenging task, as alternative splice forms are often temporal, tissue specific, and low abundant (7). So far most work has been done starting at the mRNA level, making use of the vast amount of public-domain expressed sequence tag data as well as RNA sequencing data (8 -11). Expressed sequence tags or RNA sequencing reads that belong to one gene are clustered together and then aligned with the genomic sequence in order to identify alternative splicing events. These efforts have resulted in many publically available alternative splicing databases. Most of them are generated by mining data from GenBank, UniGene and Swiss-Prot. The Evidence