Scientists have found that a specific defect in the male sex (Y) chromosome, known to cause azoospermia, or the inability to make sperm, can also cause the most common form of male infertility-low sperm production, or oligozoospermia. This study is the first to definitively show that genetic defects can cause low sperm counts in some males and suggests that intracytoplasmic sperm injection (ICSI)—the now popular technology of injecting a single sperm into an egg to circumvent low sperm counts—may cause the sons of these men to inherit infertility.