BAM is the compressed binary version of the Sequence Alignment/Map (SAM) format, a compact and indexable representation of nucleotide sequence alignments. Many next-generation sequencing and analysis tools work with SAM/BAM. For custom track display, the main advantage of indexed BAM over PSL and other human-readable alignment formats is that only the portions of the files needed to display a particular region are transferred to UCSC. This makes it possible to display alignments from files that are so large that the connection to UCSC would time out when attempting to upload the whole file to UCSC. Both the BAM file and its associated index file remain on your web-accessible server (http, https, or ftp), not on the UCSC server. UCSC temporarily caches the accessed portions of the files to speed up interactive display.