Experiments have shown that elasticity of disordered filamentous networks with compliant crosslinks is very different from networks with rigid crosslinks. Here, we model and analyze filamentous networks as a collection of randomly oriented rigid filaments connected to each other by flexible crosslinks that are modeled as worm-like chains. For relatively large extensions we allow for enthalpic stretching of crosslinks' backbones. We show that for sufficiently high crosslink density, the network linear elastic response is affine on the scale of the filaments' length. The nonlinear regime can become highly nonaffine and is characterized by a divergence of the elastic modulus at finite strain. In contrast to the prior predictions, we do not find an asymptotic regime in which the differential elastic modulus scales linearly with the stress, although an approximate linear dependence can be seen in a transition from entropic to enthalpic regimes. We discuss our results in light of the recent experiments.PACS numbers: 787.16. Ka, 87.15. La, 82.35.Pq The mechanical response of animal cells is largely determined by a visco-elastic matrix known as the cytoskeleton, which is a network consisting of many different biopolymers together with various binding proteins that govern the organization and stability of these networks. One of the ways in which these networks differ from synthetic crosslinked polymer systems is the fact that many of the crosslinks are themselves highly compliant proteins, which can strongly affect the macroscopic network compliance. There have been many in vitro studies of reconstituted networks with rigid crosslinks [1][2][3][4][5][6][7][8][9][10][11][12]. By comparison, relatively few recent experimental or theoretical studies have focused on networks with compliant crosslinks [13][14][15][16][17][18][19].A model experimental system of filamentous networks with compliant crosslinks is that of F-actin networks with the highly compliant and physiological crosslink filamin. Experimental studies on such networks have demonstrated several striking elastic properties: These networks can have a linear modulus as low as 1 Pa, while able to withstand stresses as large as 100 Pa or more at strains of order 1 or less. They do so by stiffening dramatically by up to a factor of 1000 under applied shear stress. Moreover, in contrast to networks with noncompliant crosslinks, such networks can be subjected to relatively high strains 50% without rupturing [11,15]. Both the linear and nonlinear elastic properties of actinfilamin gels appear to be dramatically affected by the flexible nature of the crosslinks, resulting in novel behavior as compared to actin-networks with noncompliant crosslinks, and to synthetic polymer gels. Similar composites of rigid filaments and compliant crosslinks can be found in other systems, such as stiff DNA nanotubes * Author to whom correspondence should be addressed. Electronic mail: fcm@nat.vu.nl crosslinked by flexible DNA linkers [20], although much less is known about the nonlinear...