The RNA binding motif protein 4 genes RBM4a and RBM4b are located on human chromosome 11q13.2 and encode highly similar proteins of 363 and 359 amino acids, respectively. They contain two RNA recognition motifs (RRMs) and a retroviral-type Zn-finger. RBM4a binds RNA, is involved in alternative splicing and is also a part of the microRNA-processing RISC complex. In particular, RBM4a is involved in exon 10 inclusion of the tau protein. The function of RBM4b is unknown. With new monoclonal antibodies we show that RBM4a is detectable in virtually all tissues and cell lines tested while RBM4b was only found in kidney and liver. Both RBM4a and RBM4b are nuclear phosphoproteins with half-lives of 2.5h and 4.5h, respectively. To our knowledge, this is the first description of RBM4b protein in human tissue. In human brain, expression of RBM4a was strongly up-regulated in cerebellum as compared to forebrain.