What is the Biblical evidence that Jesus' mother was one of His disciples?

The Roman Catholic Church venerates Mary as not only having been blessed to be the mother of Christ, but also as having been born without sin (Immaculate Conception), and to have been a perpetual virgin. Protestants do not believe the former, and most do not believe the latter either. What is the Biblical basis for presuming that Mary was one of Jesus' disciples, i.e. a Christian? --- Note - this is not meant to cast aspersions on any particular belief, and only looks for evidence of her conversion and belief in her son's work on the cross, not whether she was born without sin, or was a virgin after Jesus' birth.