Biology sequence analysis is a fundamental task in computational biology, whose aim is to detect similarity/dissimilarity relationships between molecular sequences. Some alignment-free methods to analyze similarities/dissimilarities of DNA sequences have been proposed. However, there are few alignmentfree methods to analyze protein sequences. The amino acid sequence of a protein is the key to understanding its structure and function in the cell, so we present a new method to analyze protein primary sequence in this paper. The method is based on the graphical representation and conditional probability taken as the numerical characterization of the protein sequence. The demonstrable significance of the new method is that it cannot only analyze similarity/dissimilarity of protein sequences, but also provide more biological information about the protein sequences. According to the IE curve, we can compare the numbers of amino acids of the internal and external groups at different positions. Also the IA curve can be used to compare the numbers of amino acids of the internal and ambivalent groups at different positions. The EA curve can be used to compare the numbers of amino acids in the external and ambivalent groups at different positions. Therefore the three curves show the distribution of the three types of amino acids. Furthermore, the conditional probability reflected the distribution of the two adjacent amino acids. The new approach was applied to ND6 protein sequences of several species and results have shown that the introduction of hydropathy profile of amino acids into protein sequence is effectual and feasible. Filovirus Tropism: Cellular Molecules for Viral Entry In human and non-human primates, filoviruses (Ebola and Marburg viruses) cause severe hemorrhagic fever. Recently, other animals such as pigs and some species of fruit bats have also been shown to be susceptible to these viruses. While having a preference for some cell types such as hepatocytes, endothelial cells, dendritic cells, monocytes, and macrophages, filoviruses are known to be pantropic in infection of primates. The envelope glycoprotein (GP) is responsible for both receptor binding and fusion of the virus envelope with the host cell membrane. It has been demonstrated that filovirus GP interacts with multiple molecules for entry into host cells, whereas none of the cellular molecules so far identified as a receptor/co-receptor fully explains filovirus tissue tropism and host range. Available data suggest that the mucin-like region (MLR) on GP plays an important role in attachment to the preferred target cells, whose infection is likely involved in filovirus pathogenesis, whereas the MLR is not essential for the fundamental function of the GP in viral entry into cells in vitro. Further studies elucidating the mechanisms of cellular entry of filoviruses may shed light on the development of strategies for prophylaxis and treatment of Ebola and Marburg hemorrhagic fevers. Ebola virus (EBOV) and Marburg virus (MARV), classified as biosafety level 4 agents, belong to the Family Filoviridae. Whereas MARV consists of a single species, Lake Victoria Marburgvirus, there are four distinct EBOV species, including Zaire ebolavirus (ZEBOV), Sudan ebolavirus (SEBOV), Côte d'Ivoire ebolavirus (CIEBOV), Reston ebolavirus (REBOV), and the proposed new species Bundibugyo ebolavirus (BEBOV) Towner et al., 2008) (Figure 1 left). Among these, ZEBOV, first identified in 1976, seems to be the most virulent, killing approximately up to 90% of infected individuals, whereas REBOV, which was initially isolated from cynomolgus monkeys imported from the Philippines into the USA in 1989, is less pathogenic in experimentally infected non-human primates (Fisher-Hoch and McCormick, 1999) and has never caused lethal infection in humans .