ECAT1 (ES cell-associated transcript 1 protein) is a 217 amino acid protein that belongs to the KHDC1 family. The ECAT1 protein contains an atypical KH domain with amino acid changes at critical sites, suggesting that it may not bind RNA. Expression of ECAT1 appears to be maximal in germinal vesicle oocytes, it tails off through metaphase II oocytes and is undetectable following the completion of the oocyte to embryo transition. Specifically expressed in the oocytes, recent studies suggest that ECAT1 may function as a regulator of genomic imprinting in the oocyte. Defects in ECAT1 are the cause of hydatidiform mole recurrent type 2 (HYDM2), a disorder characterized by excessive trophoblast development that produces a growing mass of tissue inside the uterus at the beginning of a pregnancy. HYDM2 leads to abnormal pregnancies with no embryo, and cystic degeneration of the chorionic villi.