Creates a controllable crosshair. Isaac automatically and continuously fires towards the crosshair.
The crosshair will not appear in each room until Isaac fires. Pressing the "drop" key or button (Keyboard default: left Control key; Controller default: right trigger) will make the crosshair disappear and stop Isaac from firing tears until a firing key is pressed again.[1]
If taken with Lilith, Marked gives the stat upgrades but not the mark.
On PC, it is possible to control Marked with the mouse. On Windows, go to C:/Users/[username]/Documents/My Games/The Binding Of Isaac [Rebirth/Afterbirth/Afterbirth+/Repentance]/ and in the options.ini file, change the line that says MouseControl=0 to MouseControl=1.
If taken with Tainted Lilith, when the baby is launched, it permanently fires at the crosshair. Pressing the "drop" key or button (Keyboard default: left Control key; Controller default: right trigger) will make the crosshair disappear and cause the baby to return to Tainted Lilith
If taken with The Forgotten's body and Tainted Forgotten, it gives the stats up without any drawbacks, and allows the bone to be angled in any direction, similar to Analog Stick.
Synergies
Brimstone: Automatically charges and fires in the direction of the mark. The angle the beam is shot from Isaac will correct itself when the mark or Isaac repositions to continue hitting the mark.
Camo Undies: Isaac will stay camouflaged until the crosshair is moved. If you control Marked with the mouse (only possible on PC) Camo Undies will stay camouflaged.
Chocolate Milk: Charge and fire rate based on proximity to mark. Fully charges when farther away, charges less but fires faster when closer. Damage scales with the distance between Isaac and the crosshair.
Cursed Eye: Greatly increases the firing rate without needing to charge. The teleporting effect of the eye is removed.
Lil Brimstone: While firing, Lil Brimstone automatically charges and fires the beam immediately upon full charge, the beam will now target the crosshair.
Dead Tooth: The green aura only activates when pressing the buttons to fire Isaac's tears.
Dr. Fetus: Continuously fires bombs towards the marked spot.
Epic Fetus: Continuously fires missiles at the marked spot.
If the crosshair is dismissed before the next missile is dropped, it still drops in the crosshair’s last position.
Epiphora: Both items work as normal, with tear rate gradually increasing until Isaac turns his head, which can happen via him or the crosshair moving too out of parallel.
Eye of the Occult: Tears will always attempt to land on the crosshair, changing direction mid-air to do so.
Mr. ME!: The Mr. ME! crosshair will move constantly in the direction of the Marked crosshair relative to Isaac. It will not attempt to match with where Marked is aiming, making it difficult to control. Pressing the drop button will stop the Mr. ME! crosshair.
This item's icon shares the same icon as Zodiac but red instead of white. Isaac's sprite is also the same as Zodiac but red instead of black.
The mark on the ground is the same as used by Epic Fetus.
Bugs
Bug!
Mom's Knife + Haemolacria: Causes the crosshair to follow the camera instead of player input. In normal rooms this makes the crosshair get stuck in the middle. In larger rooms, the marker will only move when the camera moves, allowing for minimal control when aiming due to how the camera moves based on the direction Isaac is currently aiming.
The crosshair will not appear in each room until Isaac fires. Pressing the "drop" key or button (Keyboard default: left Control key; Controller default: right trigger) will make the crosshair disappear and stop Isaac from firing tears until a firing key is pressed again.[1]
+3.15 range.
Lilith, Marked gives the stat upgrades but not the mark.
Tainted Lilith, when the baby is launched, it permanently fires at the crosshair. Pressing the "drop" key or button (Keyboard default: left Control key; Controller default: right trigger) will make the crosshair disappear and cause the baby to return to Tainted Lilith
The Forgotten's body and 
Tainted Forgotten, it gives the stats up without any drawbacks, and allows the bone to be angled in any direction, similar to 
Analog Stick.
Brimstone: Automatically charges and fires in the direction of the mark. The angle the beam is shot from Isaac will correct itself when the mark or Isaac repositions to continue hitting the mark.
Camo Undies: Isaac will stay camouflaged until the crosshair is moved. If you control Marked with the mouse (only possible on PC) Camo Undies will stay camouflaged.
Chocolate Milk: Charge and fire rate based on proximity to mark. Fully charges when farther away, charges less but fires faster when closer. Damage scales with the distance between Isaac and the crosshair.
Cursed Eye: Greatly increases the firing rate without needing to charge. The teleporting effect of the eye is removed.
Lil Brimstone: While firing, Lil Brimstone automatically charges and fires the beam immediately upon full charge, the beam will now target the crosshair.
Maw Of The Void/
Montezuma's Revenge: While firing, these items charge automatically and activate immediately upon full charge.
Mom's Knife: Charge and fire rate based on proximity to mark. Fully charges when farther away, charges less but fires faster when closer.
Monstro's Lung: Automatically charges and fires in the direction of the mark.
Pointy Rib: The rib familiar will aim at the crosshair.
Quints: The spawned familiars will aim at the crosshair.
Revelation: Light beams are charged and fired at the crosshair automatically at a fixed angle from Isaac.
Rocket in a Jar: Bombs placed will blast at the crosshair, and cannot be placed normally until the crosshair disappears.
Tech X: Fires in the direction of the crosshair, with charging time proportionate to the distance of the crosshair from Isaac.
Tractor Beam: The beam is aimed at Marked's crosshair. Moving the crosshair rotates the entire beam along with any tears inside it.
Anti-Gravity: The tear hover effect is nullified, though the tear delay down is applied.
Technology/
Dead Tooth: The green aura only activates when pressing the buttons to fire Isaac's tears.
Dr. Fetus: Continuously fires bombs towards the marked spot.
Epic Fetus: Continuously fires missiles at the marked spot.
Epiphora: Both items work as normal, with tear rate gradually increasing until Isaac turns his head, which can happen via him or the crosshair moving too out of parallel.
Eye of the Occult: Tears will always attempt to land on the crosshair, changing direction mid-air to do so.
GB Bug: Unaffected by Marked.
Gnawed Leaf: Tears will stop auto-firing once Isaac is invulnerable.
The Ludovico Technique: Overrides Marked, but stat bonuses still apply.
Mr. ME!: The Mr. ME! crosshair will move constantly in the direction of the Marked crosshair relative to Isaac. It will not attempt to match with where Marked is aiming, making it difficult to control. Pressing the drop button will stop the Mr. ME! crosshair.
My Little Unicorn/
Unicorn Stump/
Taurus: Isaac's tears will still fire, but he will be unable to move the crosshair while invincible.
Spirit Sword: Overrides Marked, but stat bonuses still apply, and allows the sword to be angled in any direction, similar to 
Sprinkler: No effect on the sprinkler.
Technology 2: Laser only fires while the crosshair is moving, and only in cardinal directions.
Tiny Planet: The tears' center of orbit can be slightly modified using the crosshair.
Zodiac but red instead of white. 
Isaac's sprite is also the same as Zodiac but red instead of black.
Bug!
Haemolacria: Causes the crosshair to follow the camera instead of player input. In normal rooms this makes the crosshair get stuck in the middle. In larger rooms, the marker will only move when the camera moves, allowing for minimal control when aiming due to how the camera moves based on the direction Isaac is currently aiming.
The Binding of Isaac: Rebirth
