Towards dissecting the regulation of terminal differentiation, including growth arrest and apoptosis, myeloid differentiation primary response (MyD) genes, induced in the absence of de novo protein synthesis following induction of M1 myeloblastic leukemia cells for terminal differentiation have been isolated. MyD118 was one of the novel MyD genes cloned, subsequently observed also to be a primary response gene to TGF-beta, which induces M1 cells for growth arrest and apoptosis uncoupled from differentiation. The MyD118 encoded protein was observed to be remarkably similar to the protein encoded by Gadd45, a growth arrest and DNA damage induced gene, regulated in part by the tumor suppressor p53. Though evidence has accumulated that MyD118 functions as an important modulator of negative growth control both in hematopoietic and non-hematopoietic cells, its mechanism of action is unknown. To better understand the role(s) of MyD118 in negative growth control, we have analysed the expression and biological characteristics of the MyD118 protein, compared to the Gadd45 protein, in distinct pathways of growth arrest and apoptosis, including p53 dependent and independent pathways either coupled or uncoupled from differentiation. It is shown that MyD118 and Gadd45 differentially accumulated upon induction of distinct pathways of growth arrest and apoptosis; notably, MyD118, but not Gadd45, was induced by TGF-beta, whereas Gadd45, but not MyD118, was induced by activating wild type (wt) p53 function. It is also shown that MyD118 is a nuclear protein, which regardless of the pathway induced, predominantly localized within the cell nucleus, and interacted with the DNA replication and repair protein PCNA and the cyclin dependent kinase inhibitor P21WAF1/CIP1. MyD118 also modestly stimulated DNA repair in vitro. All of these characteristics were shared with Gadd45. Finally, it is demonstrated that MyD118, Gadd45 and p21 synergized in the suppression of colony formation by NIH3T3 cells. Taken together, these findings demonstrate that MyD118 and Gadd45 are representative of a new protein family that share remarkable functional similarities in the control of distinct pathways of negative growth, including the suppression of cellular growth and programmed cell death.