SASH1 (SAM and SH3 domain-containing protein 1), also known as PEPE1 (Proline-glutamate repeat-containing protein), is a 1247 amino acid protein that is significantly downregulated in the majority of primary breast tumor tissues, breast cancer cell lines, lung and thyroid tumors, as well as in certain colon carcinomas. It has been hypothesized that its expression is suppressed not due to mutation of the SASH1 gene, but instead via other mechanisms, such as promoter methylation.