May be involved in differentiation or function of ciliated cells (By similarity). May play a role in spermatogenesis. Interacts with alpha-tubulin. Localizes to the manchette in elongating spermatids. Predominantly perinuclear in bronchial epithelial cells but also detected in the nucleus in some primary epithelial cells and in a number of cell lines. Isoforms 1 is testis-specific where it is expressed exclusively in germ cells (at protein level). Isoform 2 is testis-specific. Isoform 3 and isoform 4 are expressed in both lung and testis. During male germ cell development, expression of isoform 1 begins at day 21, increases after day 30 and continues to increase in the adult. Expressed in elongating spermatids from steps 9-12 (at protein level). Expression in the lung increases at 16 dpc. Belongs to the SMRP1 family.