Antisense regulation of gene expression is a widespread but poorly understood mechanism of gene expression regulation. The potential role of antisense transcripts in tumorigenesis is most intriguing for functional research. Here we experimentally characterize an antisense mRNA asLZK overlapping the human MAP3K13/LZK gene that is involved in the mitogenesis-related JNK/SAPK signal transduction pathway. According to the functional annotation of the human genome, asLZK transcript (LOC647276) is expressed at the relatively high level and overrepresented in tumor samples. To our surprise, experimental study of human asLZK revealed that this sequence is not expressed, but represents a silent pseudogene of ribosomal protein L4 encoding gene RPL4. This pseudogene resulted from relatively recent retroposition of RPL4 mRNA into the first intron of MAP3K13 gene and does not participate in the regulation of MAP3K13 expression. This study stresses that, after initial in silico mapping efforts, experimental verification of the expression landscape is warranted.