Hypoxia is part of many physiological and pathophysiological processes, including cancer and cardiovascular diseases. Endothelial cells form the inner lining of blood vessels and initiate an orchestrated gene response under hypoxia, as they are the first cells sensing reduced oxygen conditions. The transcriptional response to hypoxic conditions is already well understood. In contrast, the knowledge about the posttranscriptional gene regulation is just beginning to emerge. Alternative splicing is one possibility to adapt the transcriptome to new conditions and serves as a platform of tissue specific adaptation. In this study two genes were analyzed, both are alternatively spliced under hypoxia. MAX (MYC associated factor X) is the central part in the MYC/MAX/MXD network of transcriptional regulators involved in apoptosis, differentiation and proliferation. MAX was identified as one of nine genes in an exon array analysis. Under prolonged hypoxic conditions, cells decrease their energy consumption through a decrease of proliferation rate. Hence, MYC protein is downregulated, counteracted by an upregulation of its antagonist MXD2. Until now, no regulation of MAX has been reported under hypoxia. In addition to the wildtype protein, two mRNA isoforms encoding C-terminal truncated MAX variants arise from alternative splicing. These are created either by retention of intron 4 (isoform E) or by inclusion of a cassette exon encoded within intron 4 (isoform C). Both C-terminal isoforms have been shown to enhance the transforming activity of c-MYC in vivo, whereas the wildtype reduces it. We show that during hypoxia both mRNAs encoding C-terminal truncated MAX variants are upregulated in endothelial cells, whereas the wildtype is reduced on mRNA and protein level. Upregulation of these mRNAs, however, does not lead to a detectable production of the corresponding protein isoforms. Isoform C is targeted to the nonsense-mediated decay (NMD) pathway. Isoform E encodes a highly unstable protein, with the last 36 aa inducing an extremely efficient downregulation also when attached to heterologous proteins. Therefore, the sole purpose of this intricate splicing regulation seems to be the reduction of wildtype protein level. As MAX is part of all MYC and MXD heterodimers, its reduction impacts on the activity of the whole network. Recent studies revealed, that deregulated splicing factors may impact on tumor progression. To gain detailed insights into the regulation of splicing factors, the alternative splicing of the RNA binding protein hnRNP DL (heterogeneous nuclear ribonucleoprotein D-like) was analyzed in the second part of this work. Under hypoxia, two splice variants were identified, differing in an alternative exon, located in the hnRNP DL 3’UTR. The mRNA containing exon 8 is increased upon oxygen starvation (48 h, 1% O2) in endothelial cells. This upregulated isoform is a target for the NMD. Therefore, alternative splicing during hypoxia leads to a decrease in the protein coding mRNA isoform and subsequently to reduced protein levels. Knockdown of hnRNP DL leads to reduced proliferation of endothelial cells, supporting a general role in cell proliferation. Additionally, analysis of the splicing regulation revealed that hnRNP DL regulates its own expression in a negative feedback loop. HnRNP DL does so by binding to its pre-mRNA, favoring inclusion of exon 8. Minigene analyses identified an intronic binding sequence 50 nt up- or downstream of exon 8. A crossregulation between the paralogous proteins hnRNP DL and hnRNP D was shown in HeLa cells. Hence, both proteins regulate their own expression as well as the expression of the other protein, respectively. This kind of crossregulation was observed for other hnRNPs before, but unlike shown here, the regulation was restricted to only one direction.