Early growth response protein 4 (EGR-4), also known as AT133, is a protein that in humans is encoded by the EGR4 gene. EGR-4 is a member of the early growth response (EGF) family of zinc finger transcription factors. Transcription factors EGR1, 2, and 3 also share an R1 domain, missing in EGR4, which can be bound by NAB proteins to prevent expression of EGR target genes. The EGR zinc finger motifs are designed to specifically recognize a 9 base pair consensus sequence (5′ GCG TGG GCG 3′), with each finger spanning three nucleotides. A loss of Egr4 results in a block in spermatogenesis at the early-mid pachytene spermatocyte stage leading to a significant reduction in the number of spermatozoa produced. Additionally, Erg4-null spermatozoa show abnormal morphology such as separated heads and fragmented flagellum. The block also occurred during the first wave of spermatogenesis as apoptotic germ cells were present from 14 dpp onward. The importance of EGR4 for fertility in mammals has also been demonstrated in human males with cryptorchidism.