[Truncated abstract] Erythropoietin (Epo) is a glycoprotein hormone involved in the formation of erythrocytes, by controlling survival, differentiation and proliferation. The technique of cDNA Representational Difference Analysis was utilized to investigate J2E-NR cells that demonstrate a viability response to Epo, but not differentiation or proliferation. The aim of this project was to identify genes that may be upregulated in response to Epo-induced survival; however, no change in gene expression was detected. This was most probably because any changes were below the limit of detectability for the cDNA RDA technique, or the viability effect was mediated post-transcriptionally. Next, it was decided to investigate HLS5, a putative tumour suppressor that was identified in a myeloid variant of the J2E cell line and had been shown to cause apoptosis. A number of HeLa cell lines inducible for Hls5 expression using the tet-off system were produced; despite extremely low expression, Hls5 was shown to produce marked suppression of growth and proliferation, particularly in colony assays colony size and numbers were halved for one induced clone. … A number of haemopoiesis-associated genes were downregulated (viz. globin genes and the Epo receptor gene), which suggested Hls5s role in the myeloid variant of J2E cells, may be to suppress genes expressed in the erythroid lineage. In addition, several interferon-responsive genes were decreased in cells with elevated HLS5, suggesting it may play a role in negatively regulating interferon signaling. Online databases were also searched for information on HLS5, and showed that it is significantly downregulated in liver, lung and uterine cancers, supporting the proposition that HLS5 is a tumour suppressor gene. In summary, a number of approaches were taken to identify the effects of the Hls5 protein. It appears that it strongly suppresses proliferation and that this is likely mediated through an effect on mitosis. This may also result in apoptosis of overexpressing cells. It is possible that this is the mechanism through which HLS5 exerts its potential tumour suppressor function, as a number of tumour suppressors appear to be associated with mitosis/apoptosis control. Hls5 is also likely to have other functions in haemopoietic cells, which includes downregulation of erythroid-specific genes and suppression of interferon responses.