Titin (also called connectin) is a major protein in sarcomere assembly as well as providing elastic return of the sarcomere post-contraction in cardiac and striated skeletal muscle tissues. In addition, it has been speculated that titin is associated with nuclear functions, including chromosome and spindle formation, and regulation of muscle gene expression. In the present study, a short isoform of titin was detected in a human osteoblastic cell line, MG63 cells, by both immunostaining and western blot. Confocal images of titin staining showed both cytoplasmic and nuclear localization in a punctate pattern. Therefore, we hypothesized that human titin might contain a nuclear localization signal (NLS). A functional NLS, 200-PAKKTKT-206, located in a low complexity, titin-specific region between Z2 and Z repeats, was found by sequentially deleting segments of the N-terminal sequence in conjunction with an EGFP reporter system and confirmed by site-directed mutagenesis. Overexpression of titin's amino terminal fragment (Z1Z2Zr) in human osteoblasts (MG63) increased cell proliferation by activating the Wnt / -catenin pathway. RT-PCR screens of tissue panels demonstrated that residues 1-206 were ubiquitously expressed at low levels in all tissues and cell types analyzed. Our data implicate a dual role for titin's amino terminal region, i.e. a novel nuclear function promoting cell division in addition to its known structural role for Z-line assembly.