Deletion of RP58 gene, which codes a transcriptional repressor, revealed the aplasia of cerebral cortex and cerebrum. Recently we cloned simiRP58 that has high homology to RP58 and showed that simiRP58 has transcriptional repressor activity to specific sequences. We generated anti-simiRP58 antibody for the first time and compared the expression of simiRP58 with that of RP58 in cerebellum during development. Both RP58 and smiRP58 are expressed in external granule cell layer (EGL) and internal granule cell layer (IGL). The expression level of RP58 started to increase dramatically at postnatal day 6 when granule cell start to increase rapidly and decreased at the adult. On the other hand, that of simiRP58 started to increase at embryo day 19 slightly and is maintained. Now we are trying to analyze the function of simiRP58 on developing cerebellum.