Insulin-like protein (INSL3, INSL4, INSL5, INSL6 & INSL7/H3 Relaxin) belongs to the insulin-like hormone superfamily, which encompasses insulin, relaxin, and insulin-like growth factors I (IGF1) and II (IGF2). Insulin gene superfamily hormones regulate cell growth, metabolism, and tissue-specific functions. Members of this family are characterized by a signal peptide, a B chain, a connecting C chain, and an A chain.

The insulin-like hormone INSL-3, also named relaxin-like factor (RLF) or Leydig-derived insulin-like peptide (LEY-IL), is expressed in various reproductive tissues and is regarded a marker of differentiation in human testicular Leydig cells.