Some profoundly deaf individuals without conventional linguistic input develop gestures, called “homesign,” to communicate. We examined homesign systems (HSs) used by four deaf Nicaraguan adults (ages 15-27), and evaluated whether homesigners’ mothers are potential sources for these systems. Study One measured mothers’ comprehension of descriptions of events (e.g., “A man taps a woman”) produced in homesign and spoken Spanish. Mothers comprehended spoken Spanish descriptions (produced by a hearing child) better than homesign descriptions, suggesting a greater degree of sharedness for spoken Spanish. Study Two compared the homesign comprehension of each homesigner’s mother to that of a native user of American Sign Language (ASL). ASL Signers performed better than mothers, confirming that homesign productions contain comprehensible information, to which mothers are not fully sensitive. Taken together, these results suggest that mothers are not the source of their deaf child’s HS, and add to evidence that HSs are more like language than like gesture.