Two novel isoforms of the recently described ZFM1 gene were identified by differential screening of a cDNA library obtained from a GM-CSF dependent human myeloid leukemia cell line (GF-D8). The transcription pattern of the ZFM1 gene is complex and characterized by four transcripts of approximately 3.9, 3.7, 3.2 and 2.9 kb which are apparently up-regulated upon stimulation with GM-CSF. The same pattern of expression is also observed in freshly isolated myeloid leukemia cells and human normal CD34+ stem cells. In light of these data, and since GM-CSF is known to stimulate signal transduction pathways, it become relevant that all the different isoforms of ZFM1 contain the KH module which is a sequence motif present in proteins playing a major role in regulating cellular RNA metabolism. Search for functional domains demonstrates that ZFM1 belongs to a new and growing family of genes including Sam68, a target of Src, Fyn and Grb2 and the newly clone mouse quaking proteins (QKI) necessary in early embryo-genesis and myelination. All these molecules are thought to play a down stream role in cell signaling and RNA binding and were defined as signal transduction and activator of RNA (STAR) proteins. ZFM1 is highly conserved from yeast to humans and this might imply that this new pathways has significant function.