The number of genes identified in GO Cellular Component terms (more specifically, the first level children). The bars are annotated with the number of SGD genes and the number of SGD genes in PeptideAtlas for that term. Many annotated as unknown are present in the PeptideAtlas.
King et al. Genome Biology 2006 7:R106 doi:10.1186/gb-2006-7-11-r106