The purpose of this study is to assess the effectiveness and safety of intra-amniotic administrations (injection into amniotic fluid) of an experimental drug called ER004 in male fetuses with X-linked hypohidrotic ectodermal dysplasia (XLHED). XLHED is a rare genetic disorder that affects sweat glands, skin, hair, and teeth; it is diagnosed by molecular genetic testing of the mother’s DNA. Treatment before birth (prenatal treatment) with ER004 has the potential to prevent or at least significantly reduce the severity of XLHED symptoms and may enable people living with this condition to experience an improvement in their quality of life. During normal development of a baby in the womb, the baby secretes a protein called EDA-1. This protein helps a baby’s skin, teeth, hair, and sweat glands develop properly. EDA-1 is missing or non-functioning in XLHED-affected boys. The study drug (ER004) is a protein that is grown in a laboratory and has been designed to resemble EDA-1. Given at the right time during the baby’s development in the womb, ER004 should substitute the missing EDA-1 and trigger the mechanism that leads to the normal development of a baby’s skin, teeth, hair, and sweat glands. ER004 is injected 3 times into the amniotic fluid surrounding the fetus towards the end of the second trimester and during the third trimester of pregnancy. Previous studies have validated how ER004 may work in fetuses. ER004 treatment may improve the chances of the fetus developing functional sweat glands, which are essential for regulating the body’s temperature and improving some other key XLHED symptoms (teeth, eyes glands and salivation).