DPPL1 and is a 223 amino acid protein that is localized to the cellular membrane. HTPAP is a multi-pass membrane protein which is thought to possess six transmembrane spanning domains and is expressed as two isoforms. HTPAP is a member of the PAP related phosphoesterase family and the gene encoding HTPAP is highly conserved among many species. HTPAP functions as a PAP for lipid phosphate substrates, including PA (phosphatidate), LPA (lysophatidate) and DGPP (diacylglycerol pyrophosphate), but preferentially targets DGPP. Breast cancer tumors, specifically those in which ER (estrogen receptor) is present in high amounts, exhibit upregulation of the gene which encodes HTPAP and HTPAP is thought to increase ER activity. Due to the overexpression of HTPAP in ductal breast carcinomas and the observation that, in cases of lower HTPAP expression, tumors grew slower, HTPAP is thought to be an oncogene. In contrast, the gene encoding HTPAP is downregulated in HCC (hepatocellular carcinoma) and is thought to inhibit lung metastasis.