I. RESULTS The cell differentiation process plays a crucial role in the prenatal development of multi- cellular organisms. Recent advances in the research on stem cell properties and embryonic development have uncovered several steps in the differentiation process [1–8]. Single and multiple sequences of cell differentiation have been identified through in-vivo observations of a particular embryo during early stages of development and through pathology studies of miscarriages during late stages of the process. While the identification of each cell differ- entiation step has been the subject of intense research  an integrated view of this complex process is still missing. Such a global view promises to reveal features associated with the large-scale modular organization of the cell types [5–7  9–13] with the purpose of discover- ing new functional modules between cell types using novel theoretical network analysis for community detection [10–12]. In this letter  we take advantage of the current knowledge on the sequence of cell differentiation processes  which is spread over a vast specialized litera- ture [1–6  14–28] (see the Supplementary Information SI-Table I and references therein)  to reveal and characterize the topological and dynamical features associated with the network of human cell differentiation (NHCD). We construct the NHCD by systematically gathering the scattered information on the evolution of each cell type present in the embryo and fetus from a predecessor with a higher degree of differentiation potential into a more specialized type. The process of cell differ- entiation is then mapped onto a complex network which consists of 873 nodes connected through 977 edges. The nodes in the network represent distinct cell types reported in the lit- erature [1–6  14–28] and the edges represent the association between two cell types through a differentiation event. The initial steps of the NHCD are shown in the inset of Fig. 1  while the resulting network structure is shown in the main panel of Fig. 1 (see also SI-Figs. 5a and 5b). The fertilized egg is followed by the ball stage  and the formation of the primary germ cell layers. Currently  it is known that until the ball stage  cell division is symmetric and produces further totipotent stem cells [1]. These cells then give rise to all the differentiated tissues of the organism as well as the extra-embryonic tissues (placenta  umbilical cord  etc.). Moreover  in the course of the entire process of organism formation  there is a monotonic decrease in the differentiation 3