Cytoplasmic intermediate filaments provide mechanical strength to cells. Essential protein, involved in attachment structures in epidermal cells that connect muscles to the external cuticle. Required in morphogenesis and epidermal integrity. Probable component of embryonic epidermal attachment structures. Functions in larval muscle attachment independently of ifa-2. Forms some heteromeric filaments with ifa-1, ifa-2, ifa-3 and probably ifa-4. Expressed in epidermal cells. Expressed in amphid sensory neurons, the excretory cells, the vulva, the uterus, the rectum and some neurons of the tail. Isoform a and isoform b display a similar pattern of expression. Isoform a is predominant in pharyngeal tonofilaments. RNAi-mediated knockdown results in 79% embryonic lethality and 19% larval lethality. Abnormal localization of epidermal structural protein pat-12 during embryogenesis. Belongs to the intermediate filament family.