Apoptosis, or programmed cell death, is a process where developmental or environmental stimuli activate a genetic program to implement a series of events that culminate in cell death. To study the nature of genes that are induced during the apoptotic death of myeloid precursor cells, representational difference analysis was performed using RNAs derived from 32Dcl3 myeloblastic cells that were proliferating in the presence of IL-3 and cells that were actively undergoing apoptosis as a result of interleukin 3 deprivation for 24 h. This report describes a novel gene [small unstable apoptotic protein (SUAP)] that is up-regulated in these cells after the removal of interleukin 3 and exposure to granulocyte colony stimulating factor. The protein encoded by this gene is a target of the proteasome and does not share homology with other previously characterized proteins. To further define SUAP's role in growth arrest and apoptosis, 32Dcl3 cells that ectopically express SUAP under the control of an inducible promoter were generated and tested for their ability to proliferate under conditions where SUAP expression is induced. These studies show that although the SUAP expressing cells exhibited suppressed proliferation rates, this was not attributable to alterations in cell cycle progression. Rather, SUAP appears to induce the appearance of Annexin V-positive cells, supporting a role for this protein in programmed cell death.