(B) The sequential assembly of a novel gene regulatory network required for reprogramming. Each connection (line) represents the regulatory interaction between the indicated nine TRs during the reprogramming course. The red connections indicate functional synergy between the TRs. The light grey circles shown at 18 hrs timepoint represent the TR genes that have not yet been expressed. The O/S/K/M labeled beads placed on the gene circles denote the direct O/S/K/M binding at putative gene regulatory regions as deciphered from our ChIP-seq binding profiles. O: Oct4, S: Sox2, K: Klf4 and M: c-Myc. 