EFNB1 gene is located on Xq13.1 region on chromosome X.
EFNB1 gene codes a protein termed Ephrin-B1 which allow cell-cell communication and consequently take part on the cellular migration and morphology of the developmental embryo.
Mutations in EFNB1 gene cause a complete or partial loss of the Ephrin-B1 protein function.
As the EFNB1 gene is located on the X chromosome it is an X-linked disorder, in which in general terms we would expect males to be more severely affected than females as they have two X chromosomes and could compensate the affected one. However, specifically on diseases caused by the EFNB1 gene, women show a more severe clinical presentation than men. This is thought to happen mainly because of a random inactivation of the other X chromosome in females, as well as the interaction of the altered and the normal cells, which give a worse result than the presence of only altered ones (cellular interference, see Figure 1). 
Figure 1. Cellular interference and phenotypic expression of EFNB1 pathogenic variants in females and males.