Leukemia inhibitory factor, or LIF, is an interleukin 6 class cytokine that affects cell growth by inhibiting differentiation. When LIF levels drop, the cells differentiate. Other properties attributed to the cytokine include: the growth promotion and cell differentiation of different types of target cells, influence on bone metabolism, cachexia, neural development, embryogenesis and inflammation.

LIF binds to the specific LIF receptor (LIFR-α) which forms a heterodimer with a specific subunit common to all members of that family of receptors, the GP130 signal transducing subunit. This leads to activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen activated protein kinase) cascades.

LIF is normally expressed in the trophectoderm of the developing embryo, with its receptor LIFR expressed throughout the inner cell mass. Leukemia inhibitory factor is a cytokine expressed in the uterus during the secretory phase of the menstrual cycle, as well as expressed during a normal pregnancy. Specifically, LIF is expressed in uterine endometrial glands and is under maternal control. When the fertilized zygote has reached the blastocyst stage, the stromal cells surrounding the blastocyst produce leukemia inhibitory factor, which needed for the blastocyst to implant into the uterine endometrium.