## 1 Introduction  
_Ebola_ and _marburgviruses_ are enveloped, negative single-strand RNA viruses of the _Filoviridae_ family [1]. Since the discovery of Marburg virus (MARV) in 1967 [2] and Ebola virus (EBOV) in 1976 [3], the US Centre of Disease Control has reported several epidemic outbreaks in humans and nonhuman primates [4; 5]. Despite intense world-wide research efforts, no antiviral treatments or vaccines have yet been licensed. In addition to primates, filoviruses infect pigs, dogs, duikers, and fruit bats in nature, and rodents and ferrets can be infected experimentally [6; 7; 8; 9; 10; 11; 12]. The viral glycoprotein (GP), the only viral surface protein, exclusively mediates the entry and internalization of filoviruses into cells. The precursor protein GP0 is synthesized on the endoplasmic reticulum, and cleaved in the constitutive secretory pathway into the surface unit GP1, which binds to host cell factors, and the transmembrane unit GP2, which mediates fusion of viral envelopes with endosomal membranes. Filoviruses display a broad cell tropism [13]. Almost any cell type with the notable exception of lymphocytes is susceptible to infection by authentic filoviruses in vitro [14; 15], or to transduction by retrovirus particles pseudotyped with GP [16; 17]. Moreover, immortalized cell lines cultured in suspension are resistant to filovirus entry, while cell adhesion enhances susceptibility to infection [18; 19]. Thus, the broad cell tropism observed in infected primates, where virus can be isolated from all organs but not from lymphocytes [14; 20; 21], is also recapitulated in vitro. The availability of host factors on the cell surface that interact with viral envelope GP or with envelope lipids such as phosphatidylserine (PtdSer) often determines viral cell tropism. Such virus-host interactions mediate virus attachment, and are a necessary prerequisite for virus internalization, viral fusion with host membranes, and viral genome release into the cytosol for transcription and replication [16; 22; 23]. Several plasma membrane proteins have been implicated in filovirus attachment: cellular lectins such as as sialoglycosoprotein receptor (ASGR-R), dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), liver/lymph node-specific intercellular adhesion molecule-3-grabbing non-integrin (L-SIGN), human macrophage C-type lectin specific for galactose and N-acetylglucosamine (hMGL), or liver and lymph node sinusoidal endothelial cell C-type lectin (LSectin) [24; 25; 26; 27; 28], T-cell immunoglobulin and mucin domain 1 and 4 (TIM-1, TIM-4) [29; 30], members of the TAM family (Tyro3, Axl, Mer) of receptor tyrosine kinases [31], integrin \(\alpha\)V\(\beta\)1 [32; 33], and scavenger receptor A. However, none of these factors seems to be essential for filoviral infection across cell lines. Rather, their role in cell entry is considered to be cell type dependent, and some of them may promote entry indirectly by regulating downstream processes such as macropinocytosis or GP proteolytic cleavage [34; 35; 36; 37]. In contrast, several intracellular proteins are essential for filovirus infection in all cell types studied thus far. The endosomal and lysosomal cysteine proteases cathepsin B and cathepsin L cleave GP and thereby expose its receptor binding domain [38], and the two-pore channel 1 (TPC1) and two-pore channel 2 (TPC2) mediate endolysosomal Ca\({}^{2+}\) efflux [39]. Finally, the endolysosomal cholesterol transporter Niemann-Pick C1 (NPC1) [40; 41] binds to processed GP1 [42]. The remarkable diversity of plasma membrane proteins implicated in filovirus cell entry prompted us to analyze twelve cell lines for a potential correlation of host factor expression to filovirus susceptibility. We could show that the neuroblastoma SH-SYSY cell line is specifically resistant to filovirus infection although all intracellular proteins known to be essential were expressed, and although its overall transcriptome was very similar to that of susceptible cell lines.