C4BPA and spontaneous abortion: Additionally, the mRNA levels of complement component 4-binding protein alpha (C4BPA), a key inhibitor of the complement system (Blom et al., 2004), were increased during the implantation window in women (Tapia et al., 2011), but decreased in women which suffered repeated implantation failure and unexplained recurrent spontaneous abortion (Lee et al., 2007; Tapia et al., 2008).